drivers: spi: nrfx: Prevent double nrfx_spi[m]_uninit calls

Attempt to deinitialize an nrfx driver that is not initialized results
in an assertion failure reported by the driver. And such attempt could
happen in SPI shims when the power state was switched between states
other than ACTIVE.

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