usb: musb: Remove usb_disable() check in module_init()

Removing the check to usb_disable() before registering the platform
driver allows to build this driver when !USB && USB_GADGET, to be
used in gadget-only mode.

Also, use module_platform_driver() to register the platform driver.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>