rt2x00: Cleanup chip handling helper functions.

Let each of them take a struct rt2x00_dev pointer as argument instead of
a mixture of struct rt2x00_chip and struct rt2x00_dev pointers.
Preparation for further clean ups in the rt2x00 chip handling, especially
for rt2800 devices.

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