igb: only disable/enable interrupt bits for igb physical function

The igb_irq_disable/enable calls were causing virtual functions associated
with the igb physical function to have their interrupts disabled.  In order
to prevent this from occuring we should only clear/set the bits related to
the physical function.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>