macintosh/via-macii: Handle /CTLR_IRQ signal correctly

I'm told that the /CTLR_IRQ signal from the ADB transceiver gets
interpreted by MacOS to mean SRQ, bus timeout or end-of-packet depending
on the circumstances, and that Linux's via-macii driver does not
correctly interpret this signal.

Instead, the via-macii driver interprets certain received byte values
(0x00 and 0xFF) as signalling end of packet and bus timeout
(respectively). Problem is, those values can also appear under other
circumstances.

This patch changes the bus timeout, end of packet and SRQ detection logic
to bring it closer to the logic that MacOS reportedly uses.

Fixes: 1da177e4 ("Linux-2.6.12-rc2") # v5.0+
Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/6541fda1d8db3ae87c3abe17d189a10dc96e2382.1593318192.git.fthain@telegraphics.com.au