iwlwifi: signedness bug in iwl_trans_pcie_tx_agg_alloc()

unsigned shorts and unsigned chars are never == -1.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>