mtg: docg3: use free_bch() instead of kfree()

Use free_bch() instead of kfree() to free init_bch()
allocated data.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>