ASoC: omap-mcbsp: Fix latency value calculation for pm_qos

The latency number is in usec for the pm_qos. Correct the calculation to
give us the time in usec

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