iwlagn: overwrite EEPROM chain setting for 6250 devices

6250 2x2 devices have 2 tx chain and 2 rx chain. For some reason,
the EEPROM contain incorrect information and indicate it only has single
tx chain. overwrite it with .cfg parameter to make sure both chain 'A' and
chain 'B' can be used for transmit and receive

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