staging: rtl8188eu: remove prototype for non-existing function

There's no usb_read_port_cancel function in this driver. Remove its
prototype.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210408195601.4762-2-martin@kaiser.cx


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