mwifiex: remove set_channel cfg80211 hook

Since mwifiex only supports managed and IBSS
modes, it doesn't need a set_channel call. The
callback might be called, but it won't matter
as nothing can use the channel.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>