drivers: flash: spi-nor: correct index into array property value

The dpd-wake-sequence has a value that is an array of three integers,
which was formerly indexed by a suffix on the property name.  This was
updated to new accessors but failed to separate the index from the
property name.  Update the access idiom.

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