usb: musb: make modules behave better

There's really no point in doing all that
initcall trickery when we can safely let
udev handle module probing for us.

Remove all of that trickery, by moving everybody
to module_init() and making proper use of
platform_device_register() rather than
platform_device_probe().

Tested-by: Rajashekhara, Sudhakar <sudhakar.raj@ti.com>
Tested-by: Tasslehoff Kjappfot <tasskjapp@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>