bnxt_en: Get the full 48-bit hardware timestamp periodically

From the bnxt_timer(), read the 48-bit hardware running clock
periodically and store it in ptp->current_time.  The previous snapshot
of the clock will be stored in ptp->old_time.  The old_time snapshot
will be used in the next patches to compute the RX packet timestamps.

v2: Use .do_aux_work() to read the timer periodically.

Reviewed-by: Edwin Peer <edwin.peer@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>