staging: wfx: replace wfx_tx_queues_get_after_dtim() by wfx_tx_queues_has_cab()

staging: wfx: replace wfx_tx_queues_get_after_dtim() by wfx_tx_queues_has_cab()

It is not necessary to return a skb. Just getting the information if
there is traffic to be sent after DTIM is sufficient.

In add, the acronym "cab" (Content After (DTIM) Beacon) is used in
mac80211 to designate this kind of traffic.

So, make wfx_tx_queues_get_after_dtim() return a boolean and rename
accordingly.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200401110405.80282-21-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>