iwlwifi: transport's tx_agg_alloc must not sleep

The annotation/documentation is wrong, we call
it in a context that can't sleep.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>