staging: comedi: s626: remove MC_ENABLE macro

This macro relies on a local variable having a specific name. Replace
it with a new helper function, s626_mc_enable().

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>