ath9k : Fix for displaying the channel number

In the ath9k debugging feature 'wiphy' the current channel used by the
station is incorrectly displayed.This is because the channels available
are sequentially mapped from numbers 0 to 37.This mapping cannot be
changed as the channel number is also used as an array index
          This fix solves the above problem by calculating the channel
number from center frequency.

Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>