brcmfmac: assure bcdc dcmd api does not return value > 0

The protocol layer api defines callbacks for dongle commands.
Although not really well documented these should only return an
error code in case of an error, or 0 upon success. In the bcdc
protocol it can return value above 0 and we carry a fix in the
caller of the protocol layer api. This patch makes it adhere to
the intent of the api as described above.

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>