apm821xx: NETGEAR WNDR4700: Fix compat version

apm821xx: NETGEAR WNDR4700: Fix compat version

The definition for the netgear_wndr4700 had two different
DEVICE_COMPAT_VERSION definitions.

In commit 5815884c3a2 ("apm821xx: migrate to DSA"), an additional
DEVICE_COMPAT_VERSION := 3.0 attribute was added to the device
definition. The old one with version 2.0 stayed and was defined later
overwriting the new one.

Replace the old version 2.0 with the new version 3.0

Fixes: 5815884c3a2a ("apm821xx: migrate to DSA")
Link: https://forum.openwrt.org/t/openwrt-24-10-0-rc6-sixth-release-candidate/222466/43
Link: https://github.com/openwrt/openwrt/pull/17741
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>