drivers: udc_dwc2: Avoid unnecessary context switches

DWC2 thread must acquire UDC mutex before accessing shared resources
(peripheral registers and software data structures). Whenever software
enqueues a buffer, the caller first obtains mutex, adds the buffer to
the list, posts event to wake up DWC2 thread and releases mutex. If DWC2
thread has higher priority than the task currently holding a mutex,
there will be two completely unnecessary task switches: DWC2 will switch
in, try to obtain mutex, and then the control will be returned to the
mutex holder.

Avoid the unnecessary task switches by locking scheduler prior to
obtaining the mutex and unlocking scheduler after releasing the mutex.

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