usb: musb: avoid NULL pointer dereference

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>