sky2: reduce default Tx ring size

The default Tx ring size for the sky2 driver is quite large and could
cause excess buffer bloat for many users. The minimum ring size
possible and still allow handling  the worst case packet on 64bit platforms
is 38 which gets rounded up to a power of 2. But most packets only require
a couple of ring elements.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>