brcmfmac: Simplify and fix usage of brcmf_ifname.

brcmf_ifname is a debug function to return a name related to an ifp,
but is using a rather complex implementation. It was also used
wrongly from bcdc as it did not use the bsscfgidx as it was supposed
to, but bssidx. This patch fixes that bug and simplifies
brcmf_ifname.

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>