[axgbe] fix compiler warnings when compiling using gcc-6.4

[adrian]

Summary

This fixes some compiler warnings:

• uninitialised variable use
• Using XGBE_ADV rather than XGBE_ADV_SET in the phy-v2 path

Test Plan

• just compile tested; I don't have hardware to test, esp the phy-v2 path

[kevans]

Adding submitter, @rajesh1.kumar_amd.com -- looks like Adrian already worked up a patch here.

[rajesh1.kumar_amd.com]

Thanks @adrian for taking on this. From a compilation perspective, Looks good. I don't have any more specific comments.

[adrian]

who has test hardware that can test this and report back?

[rpokala]

I don't have the hardware to test, but the change looks fine.

1. The zero-initialization is obviously valid
2. It's clear in context that XGBE_SET_ADV() (which sets the flag) is what was really intended, instead of testing the flag with XGBE_ADV() and throwing away the result.