tty: n_gsm: Add raw-ip support

This patch adds the ability to open a network data connection over a mux
virtual tty channel. This is for modems that support data connections
with raw IP frames instead of PPP. On high speed data connections this
eliminates a significant amount of PPP overhead. To use this interface,
the application must first tell the modem to open a network connection on
a virtual tty. Once that has been accomplished, the app will issue an
IOCTL on that virtual tty to create the network interface. The IOCTL will
return the index of the interface created.

The two IOCTL commands are:

    ioctl( fd, GSMIOC_ENABLE_NET );

    ioctl( fd, GSMIOC_DISABLE_NET );

Signed-off-by: Russ Gorby <russ.gorby@intel.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>