ASoC: Blackfin: keep better track of SPORT configuration state

Do not let the SPORT be reconfigured until there are no more active
streams.  Then we can let the system reprogram the SPORT state.

Signed-off-by: Cliff Cai <cliff.cai@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>