dmaengine: shdma: (cosmetic) simplify a static function

dmae_find_slave() needs only the slave_id field from the slave object, no
need to pass the pointer to the object, pass the slave_id directly.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>