flash: nor: ath79: fix build failure due to recent MIPS changes

Change-Id: I7139b0658f048afea2d16216c93e8946356a630d
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/4151


Tested-by: jenkins
Reviewed-by: Salvador Arroyo <sarroyofdez@yahoo.es>