igb: Change to use statically allocated array for MSIx entries

This patch changes how the driver initializes MSIx and checks
for MSIx configuration.  This change makes it easier to reconfigure the
device when queue changes happen at runtime using ethtool's set_channels
feature.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>