ath9k_htc: keep calibrated noise floor value for oper channel

The ath9k_hw assumes that caldata is valid only for
oper channel. But with ath9k_htc case, the caldata is
passed for all channels on hw_reset though we are not doing
calibration on that channel. So the oper channel's nf history
got cleared to default due to mismatch in channel flags.
This patch also saves some space.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>