ena-com: remove CSUM_OFFLOAD from the plat file

CSUM_OFFLOAD is a constant which is used only by the core driver code.
Because of that it shouldn't be defined in the platform file, as it's
not the platform specific code which is used only by the ena_com layer.

Submitted by: Dawid Gorecki <dgr@semihalf.com>
Submitted by: Michal Krawczyk <mk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.