staging: comedi: s626: specify bitshift for encoder A clock source

When setting the clock source for one of the 'A' encoders to operate in
"counter" mode in `s626_set_mode_a()`, bitshift the clock source value by
`S626_CRABIT_CLKSRC_A` for consistency with the other modes.  This has
no effect on the value since `S626_CRABIT_CLKSRC_A` is 0.

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>