mvebu: fix default EU regdomain for Linksys WRT AC devices

mvebu: fix default EU regdomain for Linksys WRT AC devices

The mwlwifi driver sets the default country code for EU (fi-
rmware region code 0x30) certified devices to FR (France),
not DE (Germany). Whilst this is a trivial fix, novice users
may not know how mwlwifi negatively reacts to a non-matching
country code and may leave the setting alone. Especially si-
nce it is under the advanced settings section in LuCI.

Relevant mwlwifi driver code:
https://github.com/kaloz/mwlwifi/commit/0a550312ddb5a9e00e8d602d5571598f25a78158

The mwlwifi driver readme states "Please don't change country
code and let mwlwifi set it for you." However, OpenWrt's current
behaviour does not adhere to this with its default, 'just flashed
from factory' setting for EU devices.

Signed-off-by: Jose Olivera <oliverajeo@gmail.com>
[rebase, extend commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit d0e8b8310f7079ccf250f7eddbdf8b9d319c274d)