rt2x00: Convert rt2x00 to use generic DMA-mapping API

At the same time clean up the device administration a bit, by storing a pointer
to struct device instead of a void pointer that is dependent on the type of
device. The normal PCI and USB subsystem provided macros can be used to convert
the device pointer to the right type.
This makes the rt2x00 driver a bit more type-safe.

Signed-off-by: Gertjan van Wingerde <gwingerde@kpnplanet.nl>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>