staging: comedi: pcl818: cleanup ai_cmd()

Rename this function so it has namespace associated with the driver.

The cmd->scan_begin_src is always TRIG_FOLLOW and the only valid
cmd->convert_src is TRIG_TIMER or TRIG_EXT so pcl818_ai_cmd_mode()
is always called with a 'mode' of 1 or 3. The 'mode' actually indicates
the trigger source.

Absorb pcl818_ai_cmd_mode() into this function and simplify the code.

This also removes the need for a forward declaration.

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>