skge: fix ram buffer size calculation

This fixes problems with transmit hangs on older fiber based SysKonnect boards.

Adjust ram buffer sizing calculation to make it correct on all boards
and make it like the code in sky2 driver.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>