usb: fix musb gadget to enable OTG mode conditionally

The musb driver is usable in host, gadget or dual role mode depending
on the kernel configuration.

However, the musb gadget part of the driver is enabling OTG mode
whether the driver is built for dual role or gadget only mode. This
induces a bug for gadget only USB device controllers where the kernel
tries to use Host Negotiation Protocol with such controllers, which
causes a panic.

This behaviour is now fixed by enabling OTG mode only when musb driver
is built for dual role mode.

Signed-off-by: Apelete Seketeli <apelete@seketeli.net>
Signed-off-by: Felipe Balbi <balbi@ti.com>