[PATCH] rtl8187: ensure priv->hwaddr is always valid

conf->mac_addr is not guaranteed to be set. This ensures priv->hwaddr is
always set to a valid mac address. Thanks to Johannes Berg
<johannes@sipsolutions.net> for finding this problem.

Signed-off-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>