staging: r8188eu: use ieee80211 helper to check for beacon

Use the ieee80211_is_beacon helper to check if the incoming frame is a
beacon frame.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220515155401.352742-2-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>