usb: musb: remove references to default_a of struct usb_otg

musb drivers do not use the otg fsm framework, so referencing to
otg->default_a doesn't have any effect, so remove the references.

But tusb6010 glue driver uses it locally to control the vbus power, so
keep the references in tusb6010 only.

Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>