modules: nrf_wifi: Implement QSPI deinit

By default QSPI is uninitialized after every transaction, so, deinit is
a no-op, but if QSPI_LOW_POWER is disabled, then we need to uninitialize
the QSPI in the deinit.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>