ixgbe: fix end of loop test in ixgbe_set_vf_macvlan()

The list iterator in a list_for_each_entry() loop can never be NULL.
If the loop exits without hitting a break then the iterator points
to an offset off the list head and dereferencing it is an out of
bounds access.

Before we transitioned to using list_for_each_entry() loops, then
it was possible for "entry" to be NULL and the comments mention
this.  I have updated the comments to match the new code.

Fixes: c1fec890 ("ethernet/intel: Use list_for_each_entry() helper")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>