Minor code cleanup

[mogren]

What type of PR is this?

cleanup

Why do we need it:
More idiomatic go

If issue # is not available please add repro steps and logs from IPAMD/CNI showing the issue:
No issue

Testing:
Overriding the default values works, AWS_VPC_ENI_MTU=1500:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.196.XX  netmask 255.255.224.0  broadcast 192.168.223.255
        inet6 fe80::8e8:51ff:fea1:cccc  prefixlen 64  scopeid 0x20<link>

eniab6a8de41e1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet6 fe80::64da:d0ff:fe28:1932  prefixlen 64  scopeid 0x20<link>
        ether 66:da:d0:28:19:32  txqueuelen 0  (Ethernet)

Deleting the config completely, we fall back to the default:

enic440f455693: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 9001
        inet6 fe80::147d:7eff:fef0:f2ed  prefixlen 64  scopeid 0x20<link>
        ether 16:7d:7e:f0:f2:ed  txqueuelen 0  (Ethernet)

Automation added to e2e:
No

Will this break upgrades and downgrades. Has it been tested?:
No

Does this change require any CNI config changes?:
No

Does this PR introduce a user-facing change?:
No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jayanthvn]

LGTM :)