meta-ampere: bmcweb: Don't delete DefaultGateway when disabling DHCP

After adding a static IPv4 address, if patch the DHCP disabling,
then DefaultGateway is deleted. That makes the static IPv4 address
fail.

This commit is a temporary fix. The issue was opened a ticket to
github [1]. I will update the better solution after discussing it
with community maintainers.

[1] openbmc/bmcweb#294

Signed-off-by: Chanh Nguyen <[email protected]>

Author: chnguyen-ampere