ar71xx: set EU region code for TP-Link TL-WA901ND v4

There is no US firmware for the TL-WA901ND v4 yet, so we'll just
unconditionally set the EU region for now.

This makes LEDE flashable on these devices again. The format of the region
string is slightly different from the one used on the Archer C7 that is
generated by mktplinkfw (the second half of the region string is missing),
but it's similar enough to make it work.

Tested-by: Jannis Pinter <jannis@pinterjann.is>
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>