staging: sep: add missing destroy_workqueue() in sep_crypto.c

Add the missing destroy_workqueue() before return from
sep_crypto_setup() and sep_crypto_takedown().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>