USB: mct_u232: Fix incorrect TIOCMSET return

The low level helper returns 1 on success. The ioctl should however return
0. As this is the only user of the helper return, make the helper return 0 or
an error code.

Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=43009
Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>