[X/B][SRU][CVE-2018-21008] rsi: add fix for crash during assertions

[Connor Kuehl]

https://people.canonical.com/~ubuntu-security/cve/2018/CVE-2018-21008.html

>From the above link:

	"An issue was discovered in the Linux kernel before 4.16.7. A use-after-free
	can be caused by the function rsi_mac80211_detach in the file
	drivers/net/wireless/rsi/rsi_91x_mac80211.c."

Clean cherry pick for Bionic, only required an offset adjustment for Xenial.