Fix the flakiness in the CUDA ADB driver on m68k macs (keypresses getting
wedged down or ADB just going AWOL altogether).

The only IRQ used by this driver is the VIA shift register IRQ. The PowerMac
conditional code disables the other VIA IRQ sources, so don't mess with the
other IRQ flags in the common code -- m68k macs need them.

When polling, don't disable local interrupts when we only need to disable the
CUDA interrupt.

Unless polling, don't clear the shift register IRQ flag. On m68k macs this
creates a race that often breaks CUDA ADB.

Tested on Quadra 840av and LC630 (both m68k); also Beige G3 (powerpc).
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>