usb: musb: factor out hcd initalization

The musb struct is currently allocated along with the hcd, which makes
it difficult to build a driver that only acts as gadget device.

Fix this by allocating musb directly, and keep the hcd around as
a pointer in the musb struct.

struct hc_driver musb_hc_driver can now also be static to musb_host.c,
and the macro musb_to_hcd() is just a pointer dereferencer for now, and
will be eliminated later.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Felipe Balbi <balbi@ti.com>