sdiob: Forward-declare struct sdio_func

Currently this happens as a side effect of the return type of
sdio_get_function() in the expansion of BUS_ACCESSOR, but this is
fragile and can break if BUS_ACCESSOR changes to define other
functions first.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D55351

(cherry picked from commit 82b293ea6987ebcadf1142acd2218f50ab5e3fc8)