firewire: Survive more than 256 bus resets

The "color" is used during the topology building after a bus reset,
hovever in "struct fw_node"s it is stored in a u8, but in struct fw_card
it is stored in an int.  When the value wraps in one struct, but not
the other, disaster strikes.

Signed-off-by: Jay Fenlason <fenlason@redhat.com>

Fixes http://bugzilla.kernel.org/show_bug.cgi?id=10922.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>