USB: RTS/DTR signal patch for airprime driver

USB: RTS/DTR signal patch for airprime driver

I encountered some problems with the airprime driver in use with a Novatel
Merlin XU870:

Closing an open Connection to e.g. /dev/ttyUSB0 doesn't reset the
RTS/DTR lines of the Modem.  Consequently, when I use minicom to
establish a connection by "ATD*99#" the modem doesn't hang up even if i
exit minicom and so I cannot reuse the modem unless I remove it and plug
it in again.

With the attached patch, the RTS/DTR lines are resetted on a close.  The
code was mainly taken from the option.c driver.