mips32, pic32 use uint8_t in 8 bit scan function

Makes code shorter.

Change-Id: I6cc01adffbea063ccb071ddf3a3e3d81727b29ce
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/4004


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>