rt2x00: rt2800lib: update rfcsr & bbp init code for SoC devices

Update the rfcsr and bbp init code for SoC devices to match with the
latest Ralink driver.

To have better control over which values are used for the register
initialization create a new function rt2800_is_305x_soc which checks
for SoC interface type, the correct RT chipset and the correct RF
chipset. This is based on the assumption that all rt305x SoC devices
use a rt2872 and rf3020/rf3021/rf3022.

In case an unknown RF chipset is found on a SoC device with a rt2872
don't treat it as rt305x and just print a message.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>