tg3: fix ring init when there are more TX than RX channels

If TX channels are set to 4 and RX channels are set to less than 4,
using ethtool -L, the driver will try to initialize more RX channels
than it has allocated, causing an oops.

This fix only initializes the RX ring if it has been allocated.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>