ath79: add missing UBNT_REVISION

ath79: add missing UBNT_REVISION

The UBNT_REVISION was already added for the ubnt-xw target because:

  U-boot bootloader on M-XW devices expects factory image revision
  version in specific format. On airOS v6.1.7 with `U-Boot 1.1.4-s1039
  (May 24 2017 - 15:58:18)` bootloader checks if the revision major(?)
  number is actually a number, but in currently generated images there's
  OpenWrt text and so the check fails

  ...

  By placing arbitrary correct number first in major version, we make the
  bootloader happy and we can flash factory images over TFTP again.

  commit d42a7c469909 ("ath79: ubnt-m-xw: Fix factory image flashing using TFTP recovery method")

Fixes errors in the form of (tftp flashing):
  sent DATA <block=8577, 412 bytes>
  received ERROR <code=2, msg=Firmware check failed>
  Error code 2: Firmware check failed

The missing UBNT_REVISION was not noticed before, since the
UBNT_REVISION field for the ubnt-xm target was also set to:
  "42.OpenWrt-..."
Probably, UBNT_REVISION for the ubnt-xm target was set by the ubnt-xw
and was never overridden somewhere else. However, it is missing and
should be part of the ubnt-xm device.

Signed-off-by: Nick Hainke <vincent@systemli.org>