package/rtl8821cu: switch to version 20210916

The author of the 8821cu repository has requested that we switch to a newer
major version of the driver located at 8821cu-20210916:

https://github.com/morrownr/8821cu-20210118/pull/93#issuecomment-1452675897



Switch to that new repository.

Fixes a build issue against PowerPC64.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>