cfg80211: use cfg80211_wext_freq() for freq conversion

WEXT's "struct iw_freq" can also be used to handle a channel. This patch now
uses cfg80211_wext_freq() instead of hand-converting the frequency. That
allows user-space to specify channels as well, like with SIOCSIWFREQ.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>