net: aquantia: make function aq_fw2x_get_mac_permanent static

The function aq_fw2x_get_mac_permanent is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
warning: symbol 'aq_fw2x_get_mac_permanent' was not declared. Should it
be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>