iwlwifi: mvm: change sta_id to u8

The sta_id variable is used as an index in an array, should be unsigned.
Found by Klocwork.

Fixes: 9f9af3d7 ("iwlwifi: mvm: re-aggregate shared queue after unsharing")
Signed-off-by: Sharon Dvir <sharon.dvir@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>