[libomptarget] [amdgpu] Fix default setting of max flat workgroup size

When max flat workgroup size is not specified, it is set to the default
workgroup size. This prevents kernel launch with a workgroup size larger
than the default. The fix is to ignore a size of 0 and treat it as
unspecified.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D105073