ar71xx: really fix Mikrotik board detection

ar71xx: really fix Mikrotik board detection

commit e09da0169a08 ("ar71xx: fix Mikrotik board detection")
was generated based on testing a rb-912 board, on which detection failed.

Testing on more hardware shows something fun:

machine	: MikroTik RouterBOARD 922UAGS-5HPacD
machine	: Mikrotik RouterBOARD 912UAG-5HPnD

Both lowercase and uppercase are used.
So ensure we support both now ..

Fixes: e09da0169a08 ("ar71xx: fix Mikrotik board detection")
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
(cherry picked from commit 845b2a1cfe307394f3f2144cce085bbb5e171ecc)