usb: musb: avoid NULL pointer dereference

[ Upstream commit eee3f15d ]

instead of relying on the otg pointer, which
can be NULL in certain cases, we can use the
gadget and host pointers we already hold inside
struct musb.

Cc: <stable@vger.kernel.org>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Dan Murphy <DMurphy@ti.com>