staging: comedi: s626: add mmiowb() calls

`s626_mc_enable()` is often called to enable some function in a control
register after setting up some other registers.  Precede the write to
the control register with a call to `mmiowb()` to preserve mmio write
ordering.

`s626_mc_disable()` is called to disable some function in a control
register and is often followed up by writes to other registers.  Follow
the write to the control register with a call to `mmiowb()` to preserve
mmio write ordering.

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>