staging: r8188eu: use subtype helper in rtw_check_bcn_info

Use ieee80211_is_beacon to check the frame subtype in rtw_check_bcn_info.
Replace the call to the driver-specific GetFrameSubType function.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20221202093159.404111-3-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>