libertas: clean up scan.c, remove zeromac and bcastmac

Should be purely cosmetic apart from the removal of the two pointless
MAC addresses.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>