staging: comedi: s626: remove a variable from set_mode_a() and _b()

The `set_mode_a()` and `set_mode_b()` functions use a local variable
`setup` which is initialized to the value of parameter `Setup` which is
not used further.  Get rid of the local variable and rename the
parameter to `setup`.

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>