macb: restore wrap bit when performing underrun cleanup

When TX underrun occurs, a cleanup is performed that marks all buffers
as used. As a side effect it also clears the wrap bit in the last
buffer. This patch will restore the wrap bit.

Signed-off-by: Tord Andersson <tord.andersson@endian.se>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>