staging: comedi: adv_pci1710: remove 'ai_timer1' from private data

This member of the private data is just a copy of the cmd->convert_arg.
Use that instead and remove the unnecessary sanity checking since it
was already validated in the (*do_cmdtest).

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>