wifi-scripts: fix ucode mobility_domain default value

wifi-scripts: fix ucode mobility_domain default value

The mobility_domain value generated by ucode differed from the previous
shell script implementation. The legacy shell script used `echo` on the
SSID, which appended a trailing newline.

To maintain roaming compatibility with pre-25.12 releases and OpenWrt
forks in default configuration, update the ucode logic to include this
newline character when generating the default value.

Fixes: #21731
Signed-off-by: Youfu Zhang <zhangyoufu@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21732
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>