staging: comedi: das1800: change type of 'ai_speed' boardinfo

This value is compared against the unsigned int cmd->convert_arg to
check the minimum value (max speed) for the analog input conversion
timing.

For aesthetics, change the type to match the cmd->convert_arg type.

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>