ASoC: omap-hdmi-audio: Force channel allocation only for OMAP4

There is a constraint in the OMAP4 HDMI IP that requires to use
the 8-channel code when transmitting more than two channels.

The constraint doesn't apply for OMAP5 so don't force the channel
allocation in the sound driver as it can be done specifically for
OMAP4 later in the hdmi4 core.

Signed-off-by: Misael Lopez Cruz <misael.lopez@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Mark Brown <broonie@kernel.org>