iwlwifi: fix valid chain reading from EEPROM

When read valid tx/rx chains from EEPROM, there is a bug to use the
tx chain value for both tx and rx, the result of this cause low
receive throughput on 1x2 devices becuase rx will only utilize single
chain instead of two chains

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>