drivers: nrf_qspi_nor: Deactivate QSPI peripheral after initialization

This is a follow-up to commit ea1be7f2.

After the driver performs its initialization, it needs to deactivate
the QSPI peripheral. Otherwise, the peripheral would unnecessarily
consume power until some QSPI operation is performed (and only then
it will get deactivated), what depending on the application may take
a significant amount of time.

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