ixgbe: fix use of list_for_each in ixgbe_enumerate_functions

Fix a bug in the misuse of the list_for_each macro to loop over every
entry in the bus_list. Instead of attempting to loop over the list from
a random entry point, go up to the bus and use the real list_head entry
point. This prevents the possible read or write of unallocated or
incorrectly addressed memory.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>