drivers: dai: ssp: remove unused variables

Fix compiler warnings
/zep_workspace/zephyr/drivers/dai/intel/ssp/ssp.c:2079:18: error: \
unused variable 'ssrsa' [-Werror=unused-variable]
 2079 |         uint32_t ssrsa = 0;
      |                  ^~~~~
/zep_workspace/zephyr/drivers/dai/intel/ssp/ssp.c:2078:18: error: \
unused variable 'sstsa' [-Werror=unused-variable]
 2078 |         uint32_t sstsa = 0;
      |                  ^~~~~

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>