[TG3]: Eliminate one register write in tg3_restart_ints()

The register write to register 0x68 to restart interrupts is unnecessary
as the interrupt wasn't masked in that register by the irq handler. This
will save one register write in the fast path.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>