iwlwifi: dvm: make rs_tl_get_load() return void

The return value from this function is never used, so make it return
void to avoid getting the following warning when compiling wiht W=1:

/home/luca/iwlwifi/stack-dev/drivers/net/wireless/intel/iwlwifi/dvm/rs.c: In function ‘rs_tl_turn_on_agg_for_tid’:
/home/luca/iwlwifi/stack-dev/drivers/net/wireless/intel/iwlwifi/dvm/rs.c:400:6: warning: variable ‘load’ set but not used [-Wunused-but-set-variable]
  u32 load;
      ^

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>