usb: rmnet: Add support for smd close event

smd driver can generate close event depending on modem status changes.
When close event is received, indicate data session end to usb host
and also free pending control messages.

Change-Id: If1af85c5c2f64cae6b82e5079a502c643e8a581a
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>