staging: comedi: dt2814: Clear stale AI data before operation

When performing a Comedi read instruction or setting up an asynchronous
command on the AI subdevice, clear any stale data on the A/D registers
by waiting for the Status register's BUSY bit to clear (if set) and then
if the FINISH or ERR bit is set, reading the A/D Data register twice to
clear the stale data.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20210301165757.243065-2-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>