brcmfmac: support get_channel cfg80211 callback

This is important for brcmfmac as some of released firmwares (e.g.
brcmfmac4366b-pcie.bin) may pick different channel than requested. This
has been tested with BCM4366B1 in D-Link DIR-885L.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>