brcmfmac: add extension to .get_fwname() callbacks

This changes the bus layer api by having the caller provide an
extension. With this the callback can use brcmf_fw_alloc_request()
to get the needed firmware name.

Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-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>