ravb: unmap descriptors when freeing rings

ravb: unmap descriptors when freeing rings

"swiotlb buffer is full" errors occur after repeated initialisation of a
device - f.e. suspend/resume or ip link set up/down. This is because memory
mapped using dma_map_single() in ravb_ring_format() and ravb_start_xmit()
is not released.  Resolve this problem by unmapping descriptors when
freeing rings.

Fixes: c156633f1353 ("Renesas Ethernet AVB driver proper")
Signed-off-by: Kazuya Mizuguchi <[email protected]>
[simon: reworked]
Signed-off-by: Simon Horman <[email protected]>
Acked-by: Sergei Shtylyov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>