wifi: iwlwifi: use unsigned to silence a GCC 12 warning

GCC 12 says:

drivers/net/wireless/intel/iwlwifi/mvm/sta.c:1076:37: warning: array subscript -1 is below array bounds of ‘struct iwl_mvm_tid_data[9]’ [-Warray-bounds]
 1076 |                 if (mvmsta->tid_data[tid].state != IWL_AGG_OFF)
      |                     ~~~~~~~~~~~~~~~~^~~~~

Whatever, tid is a bit from for_each_set_bit(), it's clearly unsigned.

Acked-by: Kalle Valo <kvalo@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>