ath9k: Don't read NF when chip has gone through full sleep mode

NF value may be incorrect when we read it just after the chip
has gone through a full sleep mode. Reading incorrect NF values
affects RX throughput.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>