ar71xx-generic: Mark MR1750 and OM5P-AC devices as BROKEN (#878)

The MR1750 and OM5P-AC devices are based on ath9k SoCs and an external
ath10k chip. All devices which are using ath10k should be marked as broken
due to deficits in their IBSS support.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>