tg3: fix to append hardware time stamping flags

The commit f233a976 (tg3: shows
HW time stamping support only if ptp_capable is present) didn't
append hardware flags correctly. This patch fixes it.

Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>