drivers/flash/spi_nor: Fix DPD exit

Incorrectly placed preprocessor conditions caused exit_dpd
when there has been no instance of a device with
dpd_wakeup_sequence parameter.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
Signed-off-by: Jan Tore Guggedal <jantore.guggedal@nordicsemi.no>