alx: add missing stats_lock spinlock init

Trivial fix for init time stack trace occuring in
alx_get_stats64 upon start up. Should have been part of
commit adding the spinlock:
f1b6b106 alx: add alx_get_stats64 operation

Signed-off-by: John Greene <jogreene@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>