Install the built-in macsonic interrupt handler on both IRQs when using
via_alt_mapping. Otherwise the rare interrupt that still comes from the
nubus slot will wedge the nubus.

$ cat /proc/interrupts
auto       2:      89176 via2
auto       3:     744367 sonic
auto       4:          0 scc
auto       6:     318363 via1
auto       7:          0 NMI
mac        9:     119413 framebuffer vbl
mac       10:       1971 ADB
mac       14:     198517 timer
mac       17:      89104 nubus
mac       19:         72 Mac ESP SCSI
mac       56:        629 sonic
mac       62:    1142593 ide0

Version 1 of this patch had a bug where a nubus sonic card would register
two interrupt handlers. Only a built-in sonic needs both.

Versions 2 and 3 needed some cleanups, as Raylynn Knight and Christoph
Hellwig pointed out (thanks).

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>