wifi-scripts: fix ucode erp_domain and fils_cache_id values

wifi-scripts: fix ucode erp_domain and fils_cache_id values

The ucode path generates different erp_domain and fils_cache_id
values than the legacy shell path due to three mismatches:

1. erp_domain md5 input missing trailing newline (echo adds \n)
2. erp_domain output truncated to 4 chars instead of 8 (shell
   uses head -c 8)
3. fils_cache_id md5 input missing trailing newline
4. erp_domain missing fallback to mobility_domain

Same bug pattern as mobility_domain fixed in commit b1dc2736db.

Fixes: #21768
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Joshua Klinesmith <joshuaklinesmith@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22677
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>