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>

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

arch/mips/kernel/traps.c

@@ -1498,16 +1498,13 @@ asmlinkage void do_watch(struct pt_regs *regs)
 {
 	siginfo_t info = { .si_signo = SIGTRAP, .si_code = TRAP_HWBKPT };
 	enum ctx_state prev_state;
-	u32 cause;
 
 	prev_state = exception_enter();
 	/*
 	 * Clear WP (bit 22) bit of cause register so we don't loop
 	 * forever.
 	 */
-	cause = read_c0_cause();
-	cause &= ~(1 << 22);
-	write_c0_cause(cause);
+	clear_c0_cause(CAUSEF_WP);
 
 	/*
 	 * If the current thread has the watch registers loaded, save