rt2x00: Let RF chipset decide the RF channel switch method to use in rt2800.

It seems that the distinction between RF channel switch method is solely based
on the RF chipset that is used.
Refactor the channel switch decision to just take the RF chipset into account,
thereby greatly simplifying the check.

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>