sunxi: musb: Check Vbus-det before enabling otg port power

Sending out 5V when there is a charger connected to the otg port is not a
good idea, so check for this and error out.

Note this commit currently breaks otg support on the q8h tablets, as we need
to do some magic with the pmic there to get vbus info, this is deliberate
(better safe then sorry), fixing this is on my TODO list.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>