cfg80211: fix incorrect checking of return value

Since cfg80211_chandef_dfs_required() can return error number(e.g.,
-EINVAL), it's not sufficient to just check if it didn't return zero.

Fixes: 9e0e29615a20 ("cfg80211: consider existing DFS interfaces")
cfg80211/mac80211: rename cfg80211_chandef_dfs_required()

Change the name to cfg80211_chandef_dfs_check to emphasize that the
function isn't a yes/no function, its return value must be fully
checked.

Also rename the helper function's name to
cfg80211_do_chandef_dfs_check for consistency.

Variable width needn't to be checked for sanity, since above
cfg80211_chandef_valid() already checked the sanity of chandef->width.
cfg80211: change variable name

Change "err" to "ret", since return value isn't zero doesn't always
mean error occured, sometimes it means success.

if (err)
        /* do things if function successfully returned */

looks strange to me, so change it.