drivers: i2s_cavs: fix for I2S start after stop

Fixed an issue I2S wouldn't resume when started after a stop
Added code to empty the TX/RX FIFOs upon stop
TX stop is achieved by letting FIFO underrun, then changing state in ISR
RX FIFO is read until empty when RX is stopped

Signed-off-by: Sathish Kuttan <sathish.k.kuttan@intel.com>