staging: r8188eu: remove unused prototype

Remove the prototype for wifirate2_ratetbl_inx, it is not needed.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211226212535.197989-11-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>