rt2x00: rt2800lib: separate RT and RF chipset detection

Use the newly introduced rt2x00_set_{rf,rt} helpers
to set the RT and RF chipset separately.

This change makes it possible to move the RT
detection code into another function which will
be done in a subseqent patch.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>