usb: sl811-hcd: Convert to module_platform_driver

Use the module_platform_driver macro, move the usb_disabled() check to
the probe function and get rid of the rather pointless message on module
load.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>