rt2x00: rt2800lib: add default_power3 field for three-chain devices

The actual code uses two default TX power values.
This is enough for 1T and for 2T devices however
on 3T devices another value is needed for the third
chain.

Add a new field to struct channel_info and initialize
it from the 'rt2800_probe_hw_mode' function. Also modify
the 'rt2800_config_channel' to handle the new field as
well.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>