libertas_tf: Fix warning in lbtf_rx for stats struct

Fixes linux-2.6 warning:

drivers/net/wireless/libertas_tf/main.c: In function 'lbtf_rx':
drivers/net/wireless/libertas_tf/main.c:578: warning: 'stats.antenna' is used uninitialized in this function
drivers/net/wireless/libertas_tf/main.c:578: warning: 'stats.mactime' is used uninitialized in this function

stats struct needs to be set to 0 before use.

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