iwlagn: verify that huge commands are synchronous

Since huge commands all share a single buffer,
there can only be a single one in flight at a
time since otherwise they'd overwrite each
other. This is true in the driver now, but it
seems like a possible source of bugs, so add
a test to verify that huge commands are always
sent synchronously.

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