cfg80211: don't export ieee80211_get_channel

This patch makes ieee80211_get_channel a static inline defined in
cfg80211's header file which simply calls __ieee80211_get_channel
to avoid symbol clashes with the ieee80211 code.

The problem was pointed out by David Miller, thanks!

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>