brcmfmac: only use ifidx from BDC header in brcmf_rx_frames()

In brcmf_rx_frames() the call to brcmf_fweh_process_skb() could
change the ifidx using information in the event data. This is
only different to the BDC ifidx for IF ADD event. However, the
creation of the new interface is deferred to event worker so
it does not exist. After brcmf_fweh_process_skb() it is only
used to set statistics.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>