mwifiex: simplify the code around ra_list

We don't need to check if the list is empty separately
as we could use list_first_entry_or_null to cover it.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>