e1000: fix bug with shared interrupt during reset

A nasty bug was found where an MTU change (or anything else that caused a
reset) could race with the interrupt code.  The interrupt code was entered
by a shared interrupt during the MTU change.

This change prevents the interrupt code from running while the driver is in
the middle of its reset path.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>