adb: add autopoll_blocked variable to block autopoll

adb: add autopoll_blocked variable to block autopoll

Whilst autopoll is enabled it is necessary to prevent the ADB buffer contents
from being overwritten until the host has read back the response in its
entirety.

Add adb_autopoll_block() and adb_autopoll_unblock() functions in preparation
for ensuring that the ADB buffer contents are protected for explicit ADB
requests.

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: <20200623204936.24064-16-mark.cave-ayland@ilande.co.uk>