cpm1: don't send break on TX_STOP, don't interrupt RX/TX when adjusting termios parameters

Before setting STOP_TX, set _brkcr to 0 so the SMC does not send a break
character.  The driver appears to properly re-initialize _brkcr when the
SMC is restarted.

Do not interrupt RX/TX when the termios is being adjusted; it results in
corrupted characters appearing on the line.

Cc: Vitaly Bordug <vbordug@ru.mvista.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>