flash/nrf5: fix invalid write access

Requesting a flash_write with a length of 0 results in a flash
position being written. Instead we should return 0 unless there is
some other input parameter that is invalid.

Change-Id: Ife6e6e2314bbee2bde7d0b69db3122ca00c361e0
Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>