staging: comedi: s626: correct S626_CRAMSK_CLKPOL_A macro (unused)

The counter 'A' clock polarity field in the 'CRA' register is only 1 bit
wide, but the `S626_CRAMSK_CLKPOL_A` macro shows it as 2 bits wide,
which would overlap with the counter 'A' interrupt source field.  This
is harmless as the macro isn't actually used yet, but correct it anyway
as I want to use it!

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>