USB: serial: kobil_sct: clean up tiocmset()

USB: serial: kobil_sct: clean up tiocmset()

Clean up the tiocmset() implementation by simplifying the flag check,
dropping some dev_dbg(), logging errors using dev_err() and using a
common control message call for both DTR and RTS to make the existing
logic easier to follow.

Note that the modem control lines are currently only manipulated in this
function, which therefore does not require any locking.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>