skge: FIFO Ram calculation error

The calculation of usable FIFO RAM is wrong in the skge driver.
First, is doesn't take into account the reserved area on the original
SysKonnect Genesis boards. Second it has an off-by-one error because
hw->ports is either 1 or 2.

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