GUIWindowVideoNav.cpp: Fix comparison bug

[garbear]

While tracking down compiler warnings, I came across a tautological compare that is probably a bug.

The warning given was:

GUIWindowVideoNav.cpp:1043:15: warning: comparison of unsigned enum expression >= 0 is always true [-Wtautological-unsigned-enum-zero-compare]
      if (ret >= 0)
          ~~~ ^  ~

I think the author meant to write it as in this PR.

Introduced in #3017.

Motivation and Context

Noticed when tracking down compiler warnings for #14309

How Has This Been Tested?

Untested.

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)