ixgbe: Fix use after free on module remove

While testing the TCP changes I had to fix an issue in order to be able to
load and unload the module.

The recent patch that added thermal sensor support added a use after free
bug on module unload with an 82598 adapter in the system.  To resolve the
issue I have updated the code so that when we free the info_kobj we set it
back to NULL.

I suspect there are likely other bugs present, but I will leave that for
another patch that can undergo more testing.

I am submitting this directly to net-next since this fixes a fairly serious
bug that will lock up the ixgbe module until the system is rebooted.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>