Pic32mx: make row programming work with any offset

In function pic32mx_write_block() if the parameter
offset is not a multiple of row size the row offset
(offset % row_size) will be ignored by the flash
controller, shifting the code to the beginning of
the row.
Word programming gets it right.

Change-Id: I134913e3d533688f791bbcb0c6e8983524197f3c
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/796


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>