usb: mtu3: fix panic in mtu3_gadget_stop()

usb: mtu3: fix panic in mtu3_gadget_stop()

commit 20914919ad31849ee2b9cfe0428f4a20335c9e2a upstream.

This patch fixes a possible issue when mtu3_gadget_stop()
already assigned NULL to mtu->gadget_driver during mtu_gadget_disconnect().

[<ffffff9008161974>] notifier_call_chain+0xa4/0x128
[<ffffff9008161fd4>] __atomic_notifier_call_chain+0x84/0x138
[<ffffff9008162ec0>] notify_die+0xb0/0x120
[<ffffff900809e340>] die+0x1f8/0x5d0
[<ffffff90080d03b4>] __do_kernel_fault+0x19c/0x280
[<ffffff90080d04dc>] do_bad_area+0x44/0x140
[<ffffff90080d0f9c>] do_translation_fault+0x4c/0x90
[<ffffff9008080a78>] do_mem_abort+0xb8/0x258
[<ffffff90080849d0>] el1_da+0x24/0x3c
[<ffffff9009bde01c>] mtu3_gadget_disconnect+0xac/0x128
[<ffffff9009bd576c>] mtu3_irq+0x34c/0xc18
[<ffffff90082ac03c>] __handle_irq_event_percpu+0x2ac/0xcd0
[<ffffff90082acae0>] handle_irq_event_percpu+0x80/0x138
[<ffffff90082acc44>] handle_irq_event+0xac/0x148
[<ffffff90082b71cc>] handle_fasteoi_irq+0x234/0x568
[<ffffff90082a8708>] generic_handle_irq+0x48/0x68
[<ffffff90082a96ac>] __handle_domain_irq+0x264/0x1740
[<ffffff90080819f4>] gic_handle_irq+0x14c/0x250
[<ffffff9008084cec>] el1_irq+0xec/0x194
[<ffffff90085b985c>] dma_pool_alloc+0x6e4/0xae0
[<ffffff9008d7f890>] cmdq_mbox_pool_alloc_impl+0xb0/0x238
[<ffffff9008d80904>] cmdq_pkt_alloc_buf+0x2dc/0x7c0
[<ffffff9008d80f60>] cmdq_pkt_add_cmd_buffer+0x178/0x270
[<ffffff9008d82320>] cmdq_pkt_perf_begin+0x108/0x148
[<ffffff9008d824d8>] cmdq_pkt_create+0x178/0x1f0
[<ffffff9008f96230>] mtk_crtc_config_default_path+0x328/0x7a0
[<ffffff90090246cc>] mtk_drm_idlemgr_kick+0xa6c/0x1460
[<ffffff9008f9bbb4>] mtk_drm_crtc_atomic_begin+0x1a4/0x1a68
[<ffffff9008e8df9c>] drm_atomic_helper_commit_planes+0x154/0x878
[<ffffff9008f2fb70>] mtk_atomic_complete.isra.16+0xe80/0x19c8
[<ffffff9008f30910>] mtk_atomic_commit+0x258/0x898
[<ffffff9008ef142c>] drm_atomic_commit+0xcc/0x108
[<ffffff9008ef7cf0>] drm_mode_atomic_ioctl+0x1c20/0x2580
[<ffffff9008ebc768>] drm_ioctl_kernel+0x118/0x1b0
[<ffffff9008ebcde8>] drm_ioctl+0x5c0/0x920
[<ffffff900863b030>] do_vfs_ioctl+0x188/0x1820
[<ffffff900863c754>] SyS_ioctl+0x8c/0xa0

Fixes: df2069acb005 ("usb: Add MediaTek USB3 DRD driver")
Signed-off-by: Macpaul Lin <macpaul.lin@mediatek.com>
Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/1604642069-20961-1-git-send-email-macpaul.lin@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>