ath9k: Fix NF calibration for single stream cards

Rather than using the chip ID to read only chain-0 CCA
registers and avoid reading chain-1, use the RX chainmask
instead. There are some 1-stream PCI devices based on AR9287
such as TL-WN751ND. Improper NF calibration might result in
DMA errors/timeouts.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>