disable scatter/gather for ENA with DPDK 18.08

disable scatter/gather for ENA with DPDK 18.08

The scatter/gather rxmode flag was set for ENA when building
against DPDK >= 18.08. ENA does not support this, so disable
it. It looks like enabling it was a copy/paste error.

Also, after offloads are adjusted based on whether "no-multi-seg"
is set, those configurations are overwritten by copying
port_conf_template over the port config. That should only happen
for versions of DPDK older than 18.08 because 18.08 and newer
make changes directly on the port config instead of making changes
to the template. Make the clib_memcpy() conditional on the DPDK
version being less than 18.08. After doing so, compiler
errors complain about port_conf_template being declared but not
used, so make it's declaration conditional.

Change-Id: If81980d71c379a565b51dd700b953f8c811a8703
Signed-off-by: Matthew Smith <mgsmith@netgate.com>