fbcon: remove restriction on margin color

This removes the restriction on the value range of the fbcon=margin:
parameter. The color value really depends on the driver being used.

Signed-off-by: David Lechner <david@lechnology.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
[b.zolnierkie: ported over fbcon changes]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>