iwlwifi: mvm: cleanup skb queue functions use

Use skb_queue_empty() and not skb_peek_tail() to check for
empty list.
Avoid a redundant check as well - loop will take care of it.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>