tg3: Unify max pkt size preprocessor constants

The maximum packet size that gets programmed into the standard producer
ring control block is directly related to the packet size used to
allocate packet buffers.  This patch removes the redundant preprocessor
constant.

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