Bluetooth: btmtksdio: Lookup device node only as fallback

If the device tree is properly written, the SDIO function device node
should be correctly defined, and the mmc core in Linux should correctly
tie it to the device being probed.

Only fall back to searching for the device node by compatible if the
original device node tied to the device is incorrect, as seen in older
device trees.

Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com> #
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>