tg3: Remove tp->rx_offset term when unneeded

This patch removes the tp->rx_offset term if NET_IP_ALIGN is defined to
zero.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>