adb: fix adb-mouse read length and revert disable-reg3-direct-writes workaround

adb: fix adb-mouse read length and revert disable-reg3-direct-writes workaround

Commit 84051eb400 "adb: add property to disable direct reg 3 writes" introduced
a workaround for spurious writes to ADB register 3 when MacOS 9 enables
autopoll on the mouse device. Further analysis shows that the problem is that
only a partial request is sent, and since the len parameter is ignored then
stale data from the previous request is used causing the incorrect address
assignment.

Remove the disable-reg3-direct-writes workaround and instead check the length
parameter when the write is attempted, discarding the invalid request.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Tested-by: Finn Thain <fthain@telegraphics.com.au>
Acked-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200623204936.24064-3-mark.cave-ayland@ilande.co.uk>