Add a restriction to the size of the Rx buffer

SiS driver forces the size of any Rx buffer to be a multiple of 64 bit.
I would not be surprized that it goes along with some alignment issues
which have been experienced before. So far it does not make much of a
difference (both drivers use 1536 bytes buffer).
Signed-off-by: NFrancois Romieu <romieu@fr.zoreil.com>
Signed-off-by: NJeff Garzik <jgarzik@pobox.com>