iwlwifi: mvm: move queue management into sta.c

None of these functions really need to be separate, they're all
only used in sta.c, move them there and make them static.

Fix a small typo in related code while at it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>