drivers: usb: udc_dwc2: Synchronize thread with UDC API

UDC API handlers and DWC2 driver thread share not only software
constructs, but also the underlying hardware. Ensure that any UDC API
call is not preempted by DWC2 driver thread (and vice versa) by
acquiring the lock in thread handler.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>