staging: comedi: rename comedi_buf_write_n_available

Rename the local function `comedi_buf_write_n_available()` to
`comedi_buf_write_n_unalloc()`.  It is the amount of unallocated space
available in the buffer that is available to be allocated for writing
and does not include the space that has already been allocated for
writing.  This is unlike the exported function
`comedi_buf_read_n_available()` which includes the space available to be
allocated for reading plus the space already allocated for reading.  The
new name breaks the unintentional naming symmetry (and also clears the
way for the old name to be reused for a new function).

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