rt2x00: rework RT chipset and revision determination for PCI an SOC devices.

The recent rt2800 devices are no longer really identified by their PCI
ID's, but rather by the contents of their CSR0 register. Also for the
other chipsets is the contents of this CSR0 register important.
Change the chipset determination logic to be more aligned with the rt2800
model.
Preparation for the support of rt3070 / rt3090 based 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>