iwlagn: warn about nested SYNC commands

Since there is no protection around SYNC host command mechanism, at least WARN
when collision happens between two SYNC host comamnds. I am not sure there is a
real  issue (beyond the HCMD_ACTIVE flag maintenance) with having two SYNC host
commands at the same time, but at least now, we will know about it.

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