drivers: flash: nrf_qspi_nor: improve diagnostics/behavior of erase

Erase can only succeed if the address is sector-aligned, and the span
to erase is an integer multiple of the sector size.  Validate this
before starting the process of erasing things.

Also reduce the check that the affected region is within the device
span to its minimal equivalent conditions.

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