IB/cxgb4: Fix if statement in pick_local_ip6adddrs

This fixes an if statement checking the return value of the function
get_lladdr for success in the function pick_local_ip6addrs to instead
of directly checking the return value of this call check the opposite
as get_lladdr returns zero for success which would incorrectly make
this if statement block not execute with the current if statement
check.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>