ASoC: rcar: skip disabled-SSI nodes

The current device tree representation of the R-Car SSI assumes that they
are numbered consecutively, starting from 0. Alas, this is not the case
with the R8A77995 (D3) SoC which SSI1/SSI2 aren't present. In order to
keep the existing device trees working, I'm suggesting to use a disabled
node for SSI0/SSI1. Teach the SSI probe to just skip disabled nodes.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>