ixgbe: Add a few safety nets for register writes and descriptor cleanups

There are possible times that a driver may fail to completely initialize,
due to a buggy platform or a buggy kernel.  In those cases, we'd rather
fail gracefully instead of a panic.  Add a few safety checks to some
critical paths to try and prevent a panic in these corner-case situations.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>