ASoC: Blackfin I2S: use dai state rather than local counter

Since the active field of the dai already tells us the stream activity,
the local counter variable is redundant and can be replaced.

Signed-off-by: Barry Song <barry.song@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>