staging: comedi: multiq3: introduce multiq3_set_ctrl()

According to the programming manual. the 'SH' and 'CLK' bits in the
control register need to be kept high at all times. Clarify this by
introducing a helper function to set the control register.

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