ath79: harmonize model names for Ubiquiti devices

ath79: harmonize model names for Ubiquiti devices

This harmonizes the model names for the ath79 Ubiquiti devices by
applying a few minor cosmetic adjustments:

- Removes hyphens where they are not found in the product names
  (Ubiquiti uses hyphens only for the abbreviated version names
  like UAP-AC-PRO which we don't use anyway.)

- Add (XM) suffix for DTS model strings to help with distinguishing
  them from their XW counterparts.

- Remove DEVICE_VARIANT for LAP-120 which actually was an alternate
  device name.

- Generally make DTS model names and those from generic-ubnt.mk
  more consistent.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>