drivers: flash: at45: fix page-size writing wrap

When writing buffers larger then page-size, there is already a routine
that checks wrap around and adjusts offsets, but this routine was
missing incrementing the data pointer, which would results in
rewriting the same page-size bytes over and over. This adds the proper
increment code.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>