staging: comedi: me4000: tidy up ME4000_AI_CTRL_REG bit defines

Use the BIT() marco to define the bits of this register.

For aesthetics, rename all the defines to remove the '_BIT' from the
name.

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>