ramips: fix 04_led_migration case syntax for mt7621

ramips: fix 04_led_migration case syntax for mt7621

Commit f761f4052c4 had bogus case syntax, the uci-defaults script threw
errors as a result and exited non-zero, probably didn't do what was
intended, but tried over and over since the non-zero exit prevents the
script from being deleted.

Fixes: f761f4052c41 ("ramips: mt7621: harmonize naming scheme for Mikrotik")

Signed-off-by: Russell Senior <russell@personaltelco.net>
[extend commit title, add Fixes]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>