usb: gadget: Add missing annotation for xudc_handle_setup()

Sparse reports a warning at xudc_handle_setup()

warning: context imbalance in xudc_handle_setup() - unexpected unlock

The root cause is the missing annotation at xudc_handle_setup()

Add the missing __must_hold(&udc->lock) annotation

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>