brcmfmac: fix initialization of struct cfg80211_inform_bss variable

This patch fixes a sparse warning "Using plain integer as NULL pointer"
about cfg80211_inform_bss structure initialization.

Reported-by: kbuild test robot <lkp@intel.com>
Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>