b43: use kernel api to replace b43 specific helper function

Use ieee80211_channel_to_frequency() to replace b43_channel_to_freq_{2,5}ghz(),
and remove unused b43_freq_to_channel_{2,5}ghz().

Signed-off-by: ZHAO Gang <gamerh2o@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>