dmaengine: ti: k3-psil: fix deadlock on error path

The mutex_unlock() is missed on error path of psil_get_ep_config()
which causes deadlock, so add missed mutex_unlock().

Fixes: 8c6bb62f

 ("dmaengine: ti: k3 PSI-L remote endpoint configuration")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20200408185501.30776-1-grygorii.strashko@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>