usb: musb: tusb6010: Use musb->tusb_revision instead of tusb_get_revision call.

The value of the revision is stored in musb->tusb_revision,
so don't re-read it every time.
Exporting tusb_get_revision is not needed anymore,
so the dependency loop between tusb6010 and tusb6010_omap is resolved.

Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
Signed-off-by: Felipe Balbi <balbi@ti.com>