netdev: bfin_mac: fix malformed UDP packet transmission when polling with KGDB

Writes to the DMA descriptors may sit in the internal Blackfin data buffers
and not actually be available when the DMA engine goes to fetch them.  This
does not typically happen, but when dealing with short/fast packets such as
UDP and polling KGDB, this occurs much more frequently.  Same goes for
heavy loads as seen by netperf tests or large scp transfers.  So force the
buffers to drain with SSYNC otherwise we get random malformed packets.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>