ixgbe: fix link issues and panic with shared interrupts for 82598

Fix possible panic/hang with shared Legacy interrupts by not enabling
interrupts when interface is down.

Also fixes an intermittent link by enabling LSC upon exit from ixgbe_intr()

This patch adds flags to ixgbe_irq_enable() to allow for some flexibility
when enabling interrupts.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>