ixgb: fix smp polling race condition

Moved interrupt masking to before requesting the interrupt from the OS.
Moved interrupt enable to after netif_poll_enable.  This fixes a racy
BUG() where polling would be running on another CPU at the same time
that netif_poll_enable would run.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: John Ronciak <john.ronciak@intel.com>