staging: comedi: drivers: properly validate cmd->scan_end_arg

When the async command 'scan_end_src' is TRIG_COUNT the 'scan_end_arg'
is the cmd->chanlist_len. Fix a couple open-coded drivers. Add the
check to the drivers that do not validate this command argument.

The core validates that the cmd->chanlist_len is <= s->len_chanlist.
During the post config after the attach the core also make sure that
the s->len_chanlist is initialized. For clarity, initialize the
s->len_chanlist in a couple drivers that are relying on the core to
initialize it to '1'.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>