staging: r8188eu: add extra TODO entries

Add entry for CFG80211 to reflect that it is currently being worked on
and is a desirable feature to have for this driver. Leave entries for
LIB80211 and MAC80211 in case anyone else wishes to work on them - they
can always be removed later, but MAC80211 in particular would certainly
be good to aim for. Also, include an entry for improving error handling,
as this is another goal of the cleanup effort.

Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210828000231.1458-1-phil@philpotter.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>