[PORT FROM R2] n_gsm.c: avoid race condition in network interface

[PORT FROM R2] n_gsm.c: avoid race condition in network interface

BZ: 15922

When the RIL ask (via iotcl) to delete the network interface, if a userland proc
ess is transfering, the xmit function can be called while the interface is delet
ed.
The interface is now stopped before deleting it.

Move from linux-2.6/drivers/char to linux-2.6/drivers/tty

Change-Id: If8e4ad645f6f048ec9b90eb225ef414acf009316
Orig-change-Id: I1e3c9881a7e9e50e30d9d2ac46cfdff4b758b532
Signed-off-by: Vincent Pillet <vincentx.pillet@intel.com>
Signed-off-by: Denis Robert <denis.robert@intel.com>
Reviewed-on: http://android.intel.com:8080/27176
Reviewed-by: Gross, Mark <mark.gross@intel.com>
Reviewed-by: Predon, Frederic <frederic.predon@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>