libertas_tf: remove set but not used variable 'flags'

libertas_tf: remove set but not used variable 'flags'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/marvell/libertas_tf/main.c: In function 'lbtf_rx':
drivers/net/wireless/marvell/libertas_tf/main.c:554:15: warning:
 variable 'flags' set but not used [-Wunused-but-set-variable]

It never used and can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Steve deRosier <derosier@cal-sierra.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>