MIPS: Add and use CAUSEF_WP definition

do_watch() clears bit 22 of cause without using a CAUSEF_* definition
from mipsregs.h. Add a definition for this bit (CAUSEF_WP) and make use
of it. Also use clear_c0_cause() instead of manual read/modify/write.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12728/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>