m68k: fix POLLWRNORM and POLLWRBAND

As noted in commit f5011c62c3 ("fix POLLWRNORM and POLLWRBAND on mips")
m68k uses a different definition.

arch/m68k/bits/poll.h

@@ -0,0 +1,2 @@
+#define POLLWRNORM POLLOUT
+#define POLLWRBAND 256