libertas: remove unreachable code from process_rxed_802_11_packet()

The function is only ever called if we're in rtap mode. So the bit in it
which is conditional on rtap mode seems a little superfluous.

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