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

This helper function is only called by the comedi core in comedi_buf.c
and comedi_fops.c. For aesthetic reasons, move it to comedi_buf.c and
remove the inline. Move the prototype from comedidev.h to comedi_internal.h
so it's not exposed outside the comedi core.

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>