staging: comedi: comedi_buf: reorder exported function prototypes

For aesthetic reasons, reorder the prototypes for the exported
comedi_buf_* functions in comedidev.h to follow the function
declarations in comedi_buf.c.

Also, change a couple of the return values from 'unsigned' to
'unsigned int' to match the value actually returned.

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>