e1000: remove irq_sem

irq_sem was just a hack to prevent interrupts from being enabled
unexpectedly in deep call paths.  Simply finding those call paths and
fixing them by hand results in a driver that behaves as we expect and
doesn't need the atomic at all.

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>