drivers: udc_dwc2: Disable endpoint while hibernated

When the endpoint is disabled while the core is hibernated, there are
timeouts waiting for interrupts. It is not clear how the stack should
behave when class and/or application wants to disable the endpoint while
device is suspended. The problem was originally observed when the
endpoints were disabled as a result of usbd_disable() call.

Avoid the timeouts by modifying the backup values instead of the real
registers (which are not accessible when hibernated).

Split the 32-bit txf_set variable into two 16-bit variables (txf_set and
pending_tx_flush) because maximum number of TxFIFO instances is 16.
The txf_set variable is used as-is, while the pending_tx_flush is used
to keep track of TxFIFOs that have to be flushed on hibernation exit.

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