USB: serial: belkin_sa: fix TIOCMBIS and TIOCMBIC

USB: serial: belkin_sa: fix TIOCMBIS and TIOCMBIC

Asserting or deasserting a modem control line using TIOCMBIS or TIOCMBIC
should not deassert any lines that are not in the mask.

Fix this long-standing regression dating back to 2003 when the
tiocmset() callback was introduced.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>