cxgb4vf: Turn off SGE RX/TX Callback Timers and interrupts in PCI shutdown routine

Need to turn off  SGE RX/TX Callback Timers & interrupt in cxgb4vf PCI Shutdown
routine in order to prevent crashes during reboot/poweroff when traffic is
running.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>