staging: comedi: Add kernel config for default buffer sizes

Allow the default values for the module parameters for the default
initial buffer size and default maximum buffer size to be specified in
the kernel configuration.

I'm not sure what the defaults for the defaults for the defaults should
be, but 64 KiB seems to small, so I used values suggested by Bernd Porr,
which are 2048 KiB for the default initial buffer size and 20480 for the
default maximum buffer size.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: Bernd Porr <berndporr@f2s.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>