brcmfmac: add function converting ieee80211_channel to chanspec

The firmware carries channel information in a different format
as the provided ieee80211_channel structure. Conversion is needed
when receiving requests from cfg80211 carrying ieee80211_channel
structures. This patch adds a utility function to do that.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>