ptp: ptp_clockmatrix: Demote non-kernel-doc header to standard comment

Fixes the following W=1 kernel build warning(s):

 drivers/ptp/ptp_clockmatrix.c:1408: warning: Cannot understand  * @brief Maximum absolute value for write phase offset in picoseconds
 drivers/ptp/ptp_clockmatrix.c:1408: warning: Cannot understand  * @brief Maximum absolute value for write phase offset in picoseconds
 drivers/ptp/ptp_clockmatrix.c:1408: warning: Cannot understand  * @brief Maximum absolute value for write phase offset in picoseconds
 drivers/ptp/ptp_clockmatrix.c:1408: warning: Cannot understand  * @brief Maximum absolute value for write phase offset in picoseconds
 drivers/ptp/ptp_clockmatrix.c:1408: warning: Cannot understand  * @brief Maximum absolute value for write phase offset in picoseconds

Cc: Richard Cochran <richardcochran@gmail.com>
Cc: IDT-support-1588@lm.renesas.com
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>