staging: comedi: separate out comedi_buf_* functions

Create a new file, comedi_buf.c, to hold all the comedi_async buffer
functions. Currently they are all in drivers.c and really don't have
any association with that source file.

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