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>
diff --git a/arch/mips/include/asm/mipsregs.h b/arch/mips/include/asm/mipsregs.h
index ca251f6..c08b6f7 100644
--- a/arch/mips/include/asm/mipsregs.h
+++ b/arch/mips/include/asm/mipsregs.h
@@ -390,6 +390,8 @@
 #define	 CAUSEF_IP7		(_ULCAST_(1)   << 15)
 #define CAUSEB_FDCI		21
 #define CAUSEF_FDCI		(_ULCAST_(1)   << 21)
+#define CAUSEB_WP		22
+#define CAUSEF_WP		(_ULCAST_(1)   << 22)
 #define CAUSEB_IV		23
 #define CAUSEF_IV		(_ULCAST_(1)   << 23)
 #define CAUSEB_PCI		26