bnxt_en: Move bnxt_ptp_init() from bnxt_open() back to bnxt_init_one()

It was pointed out by Richard Cochran that registering the PHC during
probe is better than during ifup, so move bnxt_ptp_init() back to
bnxt_init_one().  In order to work correctly after firmware reset which
may result in PTP config. changes, we modify bnxt_ptp_init() to return
if the PHC has been registered earlier.  If PTP is no longer supported
by the new firmware, we will unregister the PHC and clean up.

This partially reverts:

d7859afb ("bnxt_en: Move bnxt_ptp_init() to bnxt_open()")

Reviewed-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>