mac80211: fix IBSS/adhoc mode for brcmfmac

mac80211: fix IBSS/adhoc mode for brcmfmac

On systems using brmcfmac (e.g. Raspberry Pi Zero W) without this fix,
the final setup-call:
iw dev wlan0 ibss join ...

fails with returncode 161 and message:
"command failed: Not supported (-95)"

So this patch calls an explicit:
iw dev wlan0 set type ibss
just prior to the 'ibss join' command.

I have tested several ath9k and mt76xx devices
with different revisions: this patch does not harm.

please also apply to stable branch.

Signed-off-by: Bastian Bittorf <bb@npl.de>