tty: fix termios input-speed encoding

Make sure to clear the CIBAUD bits before OR-ing the new mask when
encoding the termios input baud rate.

This could otherwise lead to an incorrect input rate being reported back
and incidentally set on subsequent termios updates.

Fixes: edc6afc5 ("[PATCH] tty: switch to ktermios and new framework")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>