bnxt_en: Implement .adjtime() for PTP RTC mode

The adjusted time is set in the PHC in RTC mode.  We also need to
update the snapshots ptp->current_time and ptp->old_time when the
time is adjusted.

Cc: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Signed-off-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>