iwlwifi: dvm: take mutex when sending SYNC BT config command

commit 82e5a649453a3cf23516277abb84273768a1592b upstream.

There is a flow in which we send the host command in SYNC
mode, but we don't take priv->mutex.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1046495

Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
[bwh: Backported to 3.2:
 - Adjust filename, context
 - mutex is priv->shrd->mutex]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>