brcmfmac: add mapping for interface index to bsscfg index

Because the P2P Device interface in firmware uses the same interface
index as the primary interface we use the bsscfg index as index in the
struct brcmf_pub::iflist. However, in the data path we get the interface
index and not the bsscfg index. So we need a mapping of interface index
to bsscfg index, which can be determined upon handle adding the interface.

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