fdtput: Remove star from value_len documentation

When clang checks the documentation tags (with -Wdocumentation flag), it
reports the following warning:

    fdtput.c:70:11: error: parameter '*value_len' not found in the
    function declaration [-Werror,-Wdocumentation]
     * @param *value_len    Returns length of value encoded
              ^~~~~~~~~~
    fdtput.c:70:11: note: did you mean 'value_len'?
     * @param *value_len    Returns length of value encoded
              ^~~~~~~~~~
              value_len

As this sounds reasonable, remove the star from the documentation tag.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>