drivers: flash: nrf_qspi_nor: Prevent CPU hang when XIP is re-enabled

Add a simple non-XIP transaction before deactivating the QSPI after
a XIP transaction is performed. This prevents a CPU hang from occuring
when another XIP transaction is attempted after the QSPI is activated
again.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>