net: wireless: bcmdhd: Check evt data length before processing

WLC_E_PFN_BSSID_NET_LOST is conflict shared with WLC_E_PFN_SCAN_ALLGONE
WLC_E_PFN_SCAN_ALLGONE sends no data, this needs to be checked when processing
WLC_E_PFN_BSSID_NET_LOST

Change-Id: I04f04793d10ff1c36bb1c557aea2d12f57b14b2e
Signed-off-by: Ashwin <ashwin.bhat@broadcom.com>