gve: Set default duplex configuration to full

Current duplex mode was unset in the driver, resulting in the default
parameter being set to 0, which corresponds to half duplex. It might
mislead users to have incorrect expectation about the driver's
transmission capabilities.
Set the default duplex configuration to full, as the driver runs in
full duplex mode at this point.

Fixes: 7e074d5a ("gve: Enable Link Speed Reporting in the driver.")
Signed-off-by: Junfeng Guo <junfeng.guo@intel.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Message-ID: <20230706044128.2726747-1-junfeng.guo@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>