staging: comedi: comedi_buf: don't expose comedi_buf_read_n_allocated()

This helper function is only called in comedi_buf.c. Move it there and
make it static so it's not exposed globally.

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>