staging: comedi: me4000: use correct types for extracted chanspec values

The chanspec channel, range, and aref are unsigned int values. Use the
correct types when extracting them.

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>