drivers: flash: nrf_qspi_nor: Mark device as busy when locked

Mark the device as busy when it is locked to perform a flash operation
so that the PM subsystem does not try to suspend it. Also refuse to run
a PM action when the device is busy (in case `qspi_nor_pm_action()` is
called directly, not by the PM subsystem).

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