brcmfmac: Fix warn trace on module unload while in ibss mode

When the driver is being unloaded a situation can occur where the
wirelesss core (cfg80211) wants to remove the ibss, but the state
of brcmfmac has already been set to down. When an error is
returned in that situation then that will result in a stack
trace on removal of the wiphy object. This is avoided by
returning 0 when device is down on a leave_ibss call.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>