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

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>