drivers: flash: spi_nor: avoid coverity warning in untested code

Coverity doesn't invoke the build for this module with a configuration
that provides a non-zero delay for entering or releasing deep power
down, so it sees a check that a value known to be non-negative
magically became negative without being touched, and so screams about
dead code.

Refactor to make the adjustments unconditional which has the same
effect in any reasonable level of optimization while making it
slightly less obvious that the value may not change.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>