rt2x00: rt2800lib: fix antenna configuration for RT3593

On the RT3593 chipset, BBP register 86 must be
configured by different values based on the RX
antenna numbers.

Configure this register from the 'rt2800_config_ant'
function.

Based on the Ralink DPO_RT5572_LinuxSTA_2.6.0.1_20120629
driver.

Reference:
  RT3593_CONFIG_SET_BY_ANTENNA in include/chip/rt3593.h

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>