USB: musb: do not work if no gadget driver is loaded

On OTG and gadget-only configurations, we need a gadget driver
in order to work properly, so avoid changing operation modes
when there's no gadget driver loaded.

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>