libertas_tf: Remove duplicate struct declaration

struct lbtf_private is declared twice. One has been declared
at 157th line. Remove the duplicate.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210331023557.2804128-4-wanjiabing@vivo.com