USB: rmnet_smd: Register platform driver to retry smd_open

RMNET driver uses SMD channels to communicate with modem.
Opening of SMD channels can fail if modem is not up by
the time smd_open is called by rmnet. This driver currently
attepts to open SMD channel only once at USB cable connect
and doesn't retry incase of failure.
In such scenarios RMNET connection works only after USB cable
is plugged-in again. Hence, register a platform driver so that
RMNET can retry opening of SMD channel once modem is up or
rather corresponding SMD channel is ready to be opened.

CRs-Fixed: 333051
Change-Id: I353fb6b48b226c2a186142663839ef18907458e3
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>