serial: MPSC: Stop rx engine when CREAD cleared

Currently, the MPSC driver doesn't stop recieving characters when the CREAD
flag in termios->c_cflag is cleared.  It should.  Also, only start receiving
if its not already started.

Signed-off-by: Stephane Chazelas <stephane@artesyncp.com>
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>