rt2x00: rt2800lib: use {tx,rx}_chain_num to avoid superfluous EEPROM access

The {rx,tx}_chain_num fields of rt2x00dev->default_ant
contains the number of RX and TX chains already when the
rt2800_probe_hw_mode() function runs. Use those values
instead of parsing the EEPROM configuration values again.

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