scripts: dts: Fix wrong filename in 'include:' code in old scripts

The 'fname' parameter to merge_included_bindings(), giving the path to
the top-level binding file, was accidentally shadowed in the
'for fname in fnames:' loop. This could lead to the wrong filename being
used in error messages.

Discovered via this pylint warning:

    scripts/dts/extract_dts_includes.py:225:12: R1704: Redefining
    argument with the local name 'fname' (redefined-argument-from-local)

Improve naming a bit to fix it.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>