zorro: Move zorro_bus_type to bus-private header file

zorro_bus_type was never used outside the Zorro bus code.  Hence move it
from the public to the bus-private header file.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20200112164949.20196-6-geert@linux-m68k.org