mac80211: gracefully handle preexisting VIF

Gracefully handle cases where the to-be-created wireless interface already
exists on the system which might commonly happen with non-multi-SSID capable
wireless drivers.

This fixes commit 8301e613 which caused
previously ignored "Too many open files in system (-23)" errors to fail the
wireless setup procedure.

With the updated approach we'll still try recreating the vif after one
second if the first attempt to do so failed with ENFILE but we will now
consider the operation successfull if a second attempt still yields ENFILE
with the requested ifname already existing on the system.

Fixes FS#664, FS#704.

Suggested-by: Vittorio Gambaletta <openwrt@vittgam.net>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>