MIPS: Loongson64: env: Use str_on_off() helper in prom_lefi_init_env()

Remove hard-coded strings by using the str_on_off() helper function.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>