dmaengine: sun6i: Fix memory leaks

The sun6i_dma_prep_memcpy and sun6i_dma_prep_slave_sg functions were both
leaking the descriptor they allocated if an  error was happening after a
successful dma_pool_alloc call.

It also fixes a memleak that was happening in the scatter gather list
traversal, that was allocating as much descriptor as there was scatter gather
items, but only freeing the current descriptor if an error was to arise.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>