igb: Fix queue allocation method to accommodate changing during runtime

When changing number of queues using ethtool's set_channels during runtime,
a queue allocation could fail, which can leave the device in a down state.
In order to preserve the usability of the device in this scenario, this patch
changes the driver to allocate the  number of queues only if they have not
been allocated already. The first allocation is then done for the max number
of queues, which is the default queues for this driver.   With this change,
queue quantity changes are not subject to queue allocation failures.

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>