ath9k: Initialize NF values properly

Using AR_SREV_* macros for setting up the chip-specific NF
values will make adding support for new chips hard. Use separate
macros for each chip. Currently, AR9462 has the same value for
all NF limits.

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