e1000e: clearing interrupt timers causes descriptors to get flushed

Clearing the interrupt timers following an IMS clear has the unwanted
side-effect of flushing all descriptors immediately following a partial
write when interrupts are disabled.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>