bnxt_en: Do not initialize PTP on older P3/P4 chips

The driver does not support PTP on these older chips and it is assuming
that firmware on these older chips will not return the
PORT_MAC_PTP_QCFG_RESP_FLAGS_HWRM_ACCESS flag in __bnxt_hwrm_ptp_qcfg(),
causing the function to abort quietly.

But newer firmware now sets this flag and so __bnxt_hwrm_ptp_qcfg()
will proceed further.  Eventually it will fail in bnxt_ptp_init() ->
bnxt_map_ptp_regs() because there is no code to support the older chips.
The driver will then complain:

"PTP initialization failed.\n"

Fix it so that we abort quietly earlier without going through the
unnecessary steps and alarming the user with the warning log.

Fixes: ae5c42f0 ("bnxt_en: Get PTP hardware capability from firmware")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>