[PATCH] compilation errors in drivers/serial/mpsc.c

The following patch fix gcc 4 compilation errors in drivers/serial/mpsc.c

Signed-off-by: Lee Nicks <allinux@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>