drivers: udc_dwc2: Use spin lock for synchronization

Replace irq_lock() with spin lock which is proper synchronization
primitive that should be used. Because non-SMP implementations are
allowed to optimize spin lock to just locking interrupts the resulting
code is the same on all currently supported DWC2 targets.

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