rndis_wlan: split getting current channel to separate function

Split getting current channel channel from hardware to separate function
as this function will be needed later in patch 'pass channel info to
cfg80211_roamed()'.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>