iwlwifi: mvm: always use mac color zero

We don't really need to use different mac colors when adding mac
contexts, because they're not used anywhere.  In fact, the firmware
doesn't accept 255 as a valid color, so we get into a SYSASSERT 0x3401
when we reach that.

Remove the color increment to use always zero and avoid reaching 255.

CC: <stable@vger.kernel.org> [3.10+]
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>