wifi: brcmfmac: add support for vendor-specific firmware api

The driver is being used by multiple vendors who develop the firmware
api independently. So far the firmware api as used by the driver has
not diverged (yet). This change adds framework for supporting multiple
firmware apis. The vendor-specific support code has to provide a number
of callback operations. Right now it is only attach and detach callbacks
so no real functionality as the api is still common. This code only
adds WCC variant anyway, which is selected for all devices right now.
The vendor-specific part will be built in a separate module when the
driver is configured to be built as a module through Kconfig, ie. when
CONFIG_BRCMFMAC=m.

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@kernel.org>
Link: https://lore.kernel.org/r/20221129135446.151065-4-arend.vanspriel@broadcom.com