libertas: Fix a double free in if_spi_c2h_data()

The lbs_process_rxed_packet() frees the skb.  It didn't originally, but
we fixed it in commit f54930f3

 ("libertas: don't leak skb on receive
error").

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>