orinoco: move netdev interface creation to main driver

With the move to cfg80211 it's nice to keep the hardware operations
distinct from the interface, even though we can only support a single
interface.

This also means the driver resembles other cfg80211 drivers.

Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>