iwlagn: use iwl_get_debug_level instead of iwl_debug_level

The latter may return incomplete information.
For example, if one switched IWL_DL_TX on through sysfs, IWL_DL_TX bit would
have been set in priv->debug_level, but since iwl_alloc_traffic_mem looked at
iwl_debug_level only, it wouldn't have allocated the tx_traffic buffer.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>