vmxnet3: fix compilation when RSS is disabled

If RSS is disabled, we can ifdef out some RSS specific code. This fixes
the compile error found by Randy Dunlap.

Signed-off-by: Scott J. Goldman <scottjg@vmware.com>
Reviewed-by: Bhavesh Davda <bhavesh@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>