bnxt_en: Fix PTP capability discovery

The current PTP initialization logic does not account for firmware
reset that may cause PTP capability to change.  The valid pointer
bp->ptp_cfg is used to indicate that the device is capable of PTP
and that it has been initialized.  So we must clean up bp->ptp_cfg
and free it if the firmware after reset does not support PTP.

Fixes: 93cb62d9

 ("bnxt_en: Enable hardware PTP support")
Cc: Richard Cochran <richardcochran@gmail.com>
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>