tg3: Fix inconsistent locking for tg3_netif_start().

Every caller holds tp->lock when calling tg3_netif_start() except
tg3_io_resume().  Fix it so that it is all consistent.  The subsequent
PTP patches add tg3_ptp_resume() to tg3_netif_start() and the tp->lock
is required.

Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>