brcmfmac: initialize variable in brcmf_sdiod_regrl()

In case of an error the variable returned is uninitialized. The caller
will probably check the error code before using it, but better assure
it is set to zero.

Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>