staging: comedi: comedi_pci.c: Fix kernel-doc Return tags

Fix the 'Return' tags in the kernel-doc comments as they currently say
'Returns', which is not recognized by kernel-doc.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>