usb: gadget: at91_udc: move prepare clk into process context

usb: gadget: at91_udc: move prepare clk into process context

Commit 7628083227b6bc4a7e33d7c381d7a4e558424b6b (usb: gadget: at91_udc:
prepare clk before calling enable) added clock preparation in interrupt
context. This is not allowed as it might sleep. Also setting the clock
rate is unsafe to call from there for the same reason. Move clock
preparation and setting clock rate into process context (at91udc_probe).

Signed-off-by: Ronald Wahl <ronald.wahl@raritan.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: <stable@vger.kernel.org> # v3.17+
Signed-off-by: Felipe Balbi <balbi@ti.com>