sky2: enforce minimum ring size

The hardware has a restriction that the minimum ring size possible
is 128. The number of elements used is controlled by tx_pending and
the overall number of elements in the ring tx_ring_size, therefore it
is okay to limit the number of elements in use to a small value (63)
but still provide a bigger ring.

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