staging: comedi: mpc624: change type of private data 'ai_speed'

For aesthetics, change the type of this member to unsigned int.

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>