ar8216: flush ARL table during reset after init_globals

ar8216: flush ARL table during reset after init_globals

commit 33b72b8e0faf7a39faabece584fd6da61cd8f8df
"ar8216: adjust ATU flushing in case of link changes"
introduced portwise flushing on link down events. Now the ARL table could
be in a chaotic state after boot where ar8xxx_sw_get_arl_table looped
forever (depending on the entries collected while booting).

Fixes FS#384.

Signed-off-by: Günther Kelleter <guenther.kelleter@devolo.de>
(cherry picked from commit 237454991618e0e8b7ceb8a8a2a43fca12c1a454)
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>