net: dp83640: fix SIOCSHWTSTAMP to update the struct with actual configuration

[ Upstream commit 473309fb8372365ad211f425bca760af800e10a7 ]

From Documentation/networking/timestamping.txt:

  A driver which supports hardware time stamping shall update the
  struct with the actual, possibly more permissive configuration.

Do update the struct passed when we upscale the requested time
stamping mode.

Fixes: cb646e2b ("ptp: Added a clock driver for the National Semiconductor PHYTER.")
Signed-off-by: Sergey Organov <sorganov@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>