drivers: flash: nrf_qspi_nor: clean up lock/unlock idioms

Having a completion wait function release a lock internally only when
the operation fails is confusing.  Remove that feature, and make the
lock and unlock operations explicit and paired.

This makes it much more clear how to properly handle transactions that
require multiple calls to the HAL.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>