ACPI backlight fix for 3.18-rc8

This is a simple fix for an ACPI backlight regression introduced by
a recent commit that overlooked a corner case which should have been
taken into account.

/
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.22 (GNU/Linux)
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=vku/
-----END PGP SIGNATURE-----