staging: r8188eu: remove interrupt_handler from struct hal_ops

Remove interrupt_handler function pointer from struct hal_ops, as it
is unused.

Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210906010106.898-9-phil@philpotter.co.uk


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