libertas_tf: Drop unused variable and define

gcc-6 reports:

drivers/net/wireless/marvell/libertas_tf/main.c:30:19: error:
	'lbtf_driver_version' defined but not used

with -Werror=unused-const-variable=.

Reported-by: Fengguang Wu <fengguang.wu@intel.com> [0-day test robot]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>