[SUNGEM]: fix minor bug in sungem.h

This changes the Sun Gem Ether driver's tx ring buffer 
length to the proper constant.  Currently TX_RING_SIZE 
and RX_RING_SIZE are equal, so no malfunction occurs.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: David S. Miller <davem@davemloft.net>