staging: comedi: s626: replace S626_MULT_X? values

Replace the use of the `S626_MULT_X1`, `S626_MULT_X2` and `S626_MULT_X4`
clock multiplier values with the equivalent `S626_CLKMULT_1X`,
`S626_CLKMULT_2X` and `S626_CLKMULT_4X` values to avoid duplication.
Replace the use of `S626_MULT_X0` with a new macro
`S626_CLKMULT_SPECIAL` (this is treated specially by the
'ClkMultA'/'ClkMultB' field of the 'CRA'/'CRB' register).  Remove the
now unused `S626_MULT_X?` macros.

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>