usb: renesas_usbhs: bugfix: add .release function to gpriv->gadget.dev

usb: renesas_usbhs: bugfix: add .release function to gpriv->gadget.dev

This patch fixup below warning on device_unregister()

renesas_usbhs renesas_usbhs.1: host probed
renesas_usbhs renesas_usbhs.1: gadget probed
renesas_usbhs renesas_usbhs.1: irq request err
------------[ cut here ]------------
WARNING: at ${LINUX}/drivers/base/core.c:1)
Device 'gadget' does not have a release() function, it is broken and must be fi.
Modules linked in:
[<c000e25c>] (unwind_backtrace+0x0/0xe4) from [<c0016960>] (warn_slowpath_commo)
[<c0016960>] (warn_slowpath_common+0x4c/0x64) from [<c00169f8>] (warn_slowpath_)
[<c00169f8>] (warn_slowpath_fmt+0x2c/0x3c) from [<c0185b80>] (device_release+0x)
[<c0185b80>] (device_release+0x70/0x84) from [<c013e300>] (kobject_cleanup+0x58)
[<c013e300>] (kobject_cleanup+0x58/0x6c) from [<c01cba14>] (usbhs_mod_gadget_re)
[<c01cba14>] (usbhs_mod_gadget_remove+0x3c/0x6c) from [<c01c8384>] (usbhs_mod_p)
[<c01c8384>] (usbhs_mod_probe+0x68/0x80) from [<c01c7f84>] (usbhs_probe+0x1cc/0)
...

Cc: stable@vger.kernel.org
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>