m68k: sun3: Fix signature of sun3_get_model()

The mach_get_model() callback takes a "char *", not an
"unsigned char *".

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/c635afb4542ab54c3dbd4fa2fbed07a2665a2f4e.1694613528.git.geert@linux-m68k.org