brcmfmac: add parameter check in brcmf_c_mkiovar()

This patch assures data is only appended in output buffer when
data is not NULL and datalen is not zero.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>