riscv: pi: replace strlcat with strscpy in get_early_cmdline

riscv: pi: replace strlcat with strscpy in get_early_cmdline

Use the return value of strscpy() instead of calling strlen(fdt_cmdline)
again and return early on string truncation. Drop the explicit size
argument since early_cmdline has a fixed length, which strscpy()
determines using sizeof() when the argument is omitted.

Replace strlcat() with strscpy() to append CONFIG_CMDLINE.

Also remove the unnecessary fdt_cmdline NULL initialization.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20260504154924.141566-3-thorsten.blum@linux.dev
Signed-off-by: Paul Walmsley <pjw@kernel.org>