staging: r8188eu: don't call empty DoReserved function

Replace the DoReserved function pointer with NULL in mlme_sta_tbl. We can
skip the function call for reserved subtypes.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220417102221.466524-6-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>