tg3: shows HW time stamping support only if ptp_capable is present

Current tg3 shows hardware timestamping support for all devices
when that is true only for the hardware with PTP_CAPABLE flag
present.

Signed-off-by: Flavio Leitner <fbl@redhat.com>
Acked-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>