flash: fix sam3 page read/write address computation error

flash: fix sam3 page read/write address computation error

In at91sam3.c for Atmel SAM3 flash support, there are arithmetic errors in the functions sam3_page_read() and sam3_page_write().
Address locations are computed incorrectly due to an extra addition operation. This leads to memory locations being skipped during
flash writes and reads.

Smaller programs are written successfully into flash, with memory gaps, while larger programs of legitimate size fail because the
skipped memory is not utilized and therefore unavailable.

The changes address this condition, and have been tested with an Atmel SAM3X-EK evaluation board.

Change-Id: I9ea3b9ed0130b71cbc32b2294e31a6a2bc71b47a
Signed-off-by: Chuen Chou <zhouquan27@gmail.com>
Reviewed-on: http://openocd.zylin.com/806
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>