color: Fix another ip segfault when using --color switch

color: Fix another ip segfault when using --color switch

Commit 959f1428 ("color: add new COLOR_NONE and disable_color function")
introducing color enum COLOR_NONE, which is not only duplicite of
COLOR_CLEAR, but also caused segfault, when running ip with --color
switch, as 'attr + 8' in color_fprintf() access array item out of
bounds. Thus removing it and restoring "magic" offset + 7.

Reproduce with:
$ ip -c a

Signed-off-by: Petr Vorel <petr.vorel@gmail.com>