tg3: Optimize rx double copy test

On a PCIX bus, the 5701 has a bug which requires the driver to double
copy all rx packets.  The rx code uses the rx_offset device member as a
flag to determine if this workaround should take effect.  The following
patch will modify the rx_offset member such that this test will become
less clear.

The patch starts by integrating the workaround check into the packet
length check.  It rounds out the implementation by relaxing the
workaround restrictions if the platform has efficient unaligned
accesses.

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