Staging: comedi: comedi_fops: Change comedi_num_legacy_minors type

Change to unsigned to allow removal of negative value check in
init section. Use smaller data type since the max possible
value currently is 48.

Signed-off-by: Cheah Kok Cheong <thrust73@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>