drivers: dai: intel: ssp: Manage the DMA request enable/disable dynamically

Do not keep both DMA request enabled whenever the SSP is in use. Manage
the SSCR1_TSRE and SSCR1_RSRE bits in sync with the enabled directions.

When only playback is used there is no need to have the RX DMA request
enabled for example.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>