m68k/mac: Disentangle VIA and OSS initialization

macintosh_config->via_type is meaningless on Mac IIfx (i.e. the only
model with OSS chip), so skip the via_type switch statement.

Call oss_init() before via_init() because it is more important and
because that is the right place to initialize the oss_present flag.

On this model, bringing forward oss_init() and delaying via_init()
is no problem because those functions are independent.

The only requirement here is that oss_register_interrupts() happens
after via_init(). That is, mac_init_IRQ() happens after config_mac().

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>