cfg80211: fix wext iw_freq parsing

The function to parse a struct iw_freq has a stupid bug,
it returns NULL when the channel cannot be found at all,
but NULL is supposed to mean "auto". Fix this by checking
the return value of ieee80211_get_channel() and returning
ERR_PTR(-EINVAL) if it returned NULL (channel not found).

This fixes an issue where you could say (in IBSS mode)
	iwconfig wlan0 channel 21
and it would use channel 1 instead because that's the
first available channel with IBSS allowed (which is what
the "auto" setting uses).

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