e1000e: remove irq_sem

irq_sem can safely be removed by auditing all irq.*able sites to
make sure that interrupts don't get enabled unexpectedly when the
interface is down.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>