iwlagn: use proper good CRC threshold behaviour

New microcode versions use the good CRC threshold
field differently, as a flag, and in that case we
should set it to 1/0 instead of 1/65535 for an
active/passive scan.

The new behaviour is advertised by the uCode with
a feature flag.

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