loader: Fix compiler warnings about truncating the PCI ID path.

loader: Fix compiler warnings about truncating the PCI ID path.

My build was producing:

../src/loader/loader.c:121:67: warning: ‘%1u’ directive output may be truncated writing between 1 and 3 bytes into a region of size 2 [-Wformat-truncation=]

and we can avoid this careful calculation by just using asprintf (as we do
elsewhere in the file).

Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>