powerpc: Fix typo in breakpoint kgdb code.

Currently we are getting:
 arch/powerpc/kernel/kgdb.c: In function 'kgdb_arch_exit':
 arch/powerpc/kernel/kgdb.c:492:2: error: '__debugger_breakx_match' undeclared (first use in this function)
 arch/powerpc/kernel/kgdb.c:492:2: note: each undeclared identifier is reported only once for each function it appears in

Fix the typo.
Signed-off-by: NMichael Neuling <mikey@neuling.org>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/kgdb.c

@@ -489,6 +489,6 @@ void kgdb_arch_exit(void)
 	__debugger_bpt = old__debugger_bpt;
 	__debugger_sstep = old__debugger_sstep;
 	__debugger_iabr_match = old__debugger_iabr_match;
-	__debugger_breakx_match = old__debugger_break_match;
+	__debugger_break_match = old__debugger_break_match;
 	__debugger_fault_handler = old__debugger_fault_handler;
 }