brcmfmac: Only handle p2p_stop_device if vif is valid

In some situations it is possible that vif has been removed while
cfg80211 invokes the p2p_stop_device handler. This will result in
crash.

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>