USB: sierra: remove redundant modem-control requests

The tty-port implementation has already made sure that DTR/RTS have been
raised and lowered by calling dtr_rts so remove the redundant calls from
open and close.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>