staging: comedi: me4000: refactor 'ai_sh_nchan' boardinfo

Some of the boards supported by this driver can do analog input sample &
hold on 8 of the channels. The 'ai_sh_nchan' member of the boardinfo is
used to indicate which boards support this feature. To save a bit of space,
convert this member to a bit-field, 'can_do_sh_ai'. Note, this feature is
not currently supported.

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>