serial: MPSC: Fix coding style and whitespace issues

Fix up mpsc.c to be aligned with Documentation/CodingStyle.  Also fix up some
whitespace issues.

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>