wifi: brcmutil: use helper function pktq_empty() instead of open code

pktq_empty was added in commit 5b435de0 ("net: wireless: add brcm80211
drivers") but it is currently not being utilized in some areas. This commit
replaces the open code with the pktq_empty() function.

Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230527114954.3281-2-claudiajkang@gmail.com