staging: comedi: mf6x4: introduce a macro to select the AI channel

For aesthetics, use a macro to set the bit in the ADCTRL register that
selects an analog input channel.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
 drivers/staging/comedi/drivers/mf6x4.c | 9 +++++----
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>