libertas: fix uninitialized variable warning

Fixes:

drivers/net/wireless/libertas/rx.c: In function process_rxed_802_11_packet:
drivers/net/wireless/libertas/rx.c:354: error: radiotap_hdr.flags may be used uninitialized in this function

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>