macintosh/adb-iop: Implement SRQ autopolling

The adb_driver.autopoll method is needed during ADB bus scan and device
address assignment. Implement this method so that the IOP's list of
device addresses can be updated. When the list is empty, disable SRQ
autopolling.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Stan Johnson <userm57@yahoo.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0fb7fdcd99d7820bb27faf1f27f7f6f1923914ef.1590880623.git.fthain@telegraphics.com.au