bnxt_en: Refactor bnxt_ptp_init()

Instead of passing the 2nd parameter phc_cfg to bnxt_ptp_init().
Store it in bp->ptp_cfg so that the caller doesn't need to know what
the value should be.

In the next patch, we'll need to call bnxt_ptp_init() in bnxt_resume()
and this will make it easier.

Reviewed-by: Somnath Kotur <somnath.kotur@broadcom.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>