ixgb: remove irq_sem

ixgb can remove irq_sem by auditing all the call sites to make sure
that each of them makes sure the adapter is in the correct state
before re-enabling interrupts.  after doing this to all of our other
drivers it is becoming easier.

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>