drivers/flash/nrf_qspi_nor: corrects write-block-size to 4B

The driver is able to write with 32-bits word alignment of size
and offset, apart form sub-word writes for which it can write
part of word to aligned offset (which is a hack).

fixes #26729

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>