sis190: fix for x86_64 (bug 11509)

Corrected dma sync handling on small packets. Should fix
http://bugzilla.kernel.org/show_bug.cgi?id=11509


Note:
While this bug was reported only on x86_64, it could have affected
any architecture.

Signed-off-by: Riccardo Ghetta <birrachiara@tin.it>
Signed-off-by: David S. Miller <davem@davemloft.net>