drivers/flash: fix nRF RRAM write-length issue while MPSL in use

When radio-synchronization was on
(CONFIG_SOC_FLASH_NRF_RADIO_SYNC_NONE=n) context of writing was shifted
by 4 instead of write-length (this caused redundant, self-shortening
writes to the same block.)

write-len was unset when requested write len was less than
RRAM_MAX_WRITE_BUFFER.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Signed-off-by: Jakub Zymelka <jakub.zymelka@nordicsemi.no>