usb: musb: stuff leak of struct usb_hcd

since the split of host+gadget mode in commit 74c2e936 ("usb: musb:
factor out hcd initalization") we leak the usb_hcd struct. We call now
musb_host_cleanup() which does basically usb_remove_hcd() and also sets
the hcd variable to NULL. Doing so makes the finall call to
musb_host_free() basically a nop and the usb_hcd remains around for ever
without anowner.
This patch drops that NULL assignment for that reason.

Fixes: 74c2e936 ("usb: musb: factor out hcd initalization")
Cc: <stable@vger.kernel.org> # v3.11+
Cc: Daniel Mack <zonque@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>