staging: comedi: adv_pci1710: remove local var in pci171x_ai_cmd()

The local variable 'mode' is not necessary. We can determine the mode
by checking the cmd->convert_src and cmd->start_src. Do this instead
to clarify the code.

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>