usb: smd: Fix RTS bit clear on disconnection

RTS bit on smd control channel is cleared upon
disconnection and is not raised afterwards which
can cause DSM pool exhaustion on modem side.
This fix prevents RTS bit clearing on disconnection.

Change-Id: I1eb5fed5d8b1afd23a6b2bb36a59a4399858581f
CRs-Fixed: 441354
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>