ravb: unmap descriptors when freeing rings

ravb: unmap descriptors when freeing rings

[ Upstream commit a47b70ea86bdeb3091341f5ae3ef580f1a1ad822 ]

"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 <kazuya.mizuguchi.ks@renesas.com>
[simon: reworked]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>