soc: mediatek: mtk-dvfsrc: Change error check for DVFSRCv4 START cmd

soc: mediatek: mtk-dvfsrc: Change error check for DVFSRCv4 START cmd

In preparation for adding support for DVFSRC Version 4, change the
error check for the MTK_SIP_DVFSRC_START command in the probe
function to error out only if BIT(0) is set: this is still valid
for the previous DVFSRC versions, as those always set this bit in
a fail reply anyway.

Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>