staging: comedi: me4000: refactor 'ai_diff_nchan' boardinfo

This member of the boardinfo is only used as a flag indicating that the
board supports differential analog inputs. Convert the member to a bit-
field to save a bit of space. For aesthetics, rename the member to
'can_do_diff_ai'.

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>