ASoC: omap-mcbsp: No need to initialize max_xx_thres when it is not used

Initializing to -EINVAL is not correct as the variables are unsigned and
if buffer_size is 0 then they are not used anyway.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Tested-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Signed-off-by: Mark Brown <broonie@kernel.org>