i40e: make use of hlist_for_each_entry_continue

Replace a complex if->continue->else->break construction in
i40e_next_filter. We can simply use hlist_for_each_entry_continue
instead. This drops a lot of confusing code. The resulting code is much
easier to understand the intention, and follows the more normal pattern
for using hlist loops. We could have also used a break with a "return
next" at the end of the function, instead of return NULL, but the
current implementation is explicitly clear that when you reach the end
of the loop you get a NULL value. The alternative construction is less
clear since the reader would have to know that next is NULL at the end
of the loop.

Change-Id: Ife74ca451dd79d7f0d93c672bd42092d324d4a03
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>