staging: comedi: me_daq: tidy up control 2 register defines

Rename the bits of this register so they have association with the
register. Use the BIT macro to define the bits.

Reading the control 2 register updates the DAC registers. For
aesthetics, remove the MC_DAC_UPDATE define and just use the
ME_CTRL2_REG define to read the register.

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