staging: comedi: ke_counter: prefer using the BIT macro

Use a macro to define the clock source options. This fixes the
checkpatch.pl issue about:

CHECK: Prefer using the BIT macro

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>