mips32: fix typos

Change-Id: Ibb98fe3da68bf670a5bb83600bb49647db8a4163
Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Reviewed-on: http://openocd.zylin.com/2338
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>