USB: android: Fix platform driver registration

The sole purpose of android composite driver registering as a
platform driver is to receive a board file callback which updates
the USB serial number in a memory accessible to boot loader.
This driver is using platform_driver_probe() method to register
as a platform driver.  This API expects the platform device to
be registered before calling this API.  This driver init function
is failing on boards which does not have android platform device
registered.  As android platform device registration is optional,
use platform_driver_register API.

Change-Id: I3aa75d70f691021cb5d6ada29677a19e3ad9821b
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>