rt2x00: rt2800lib: probe RT chipset earlier

The 'rt2800_validate_eeprom' function uses the type of
the RT chipset for verifying the number of RX streams
on RT28x0 devices. However the type of the RT chipset
is not yet detected when the 'rt2800_validate_eeprom'
function is called.

Move the RT chipset detection code into a separate helper
function, and call it before rt2800_validate_eeprom.

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