lib: Fix sbi_ecall_register_extension to prevent extension IDs overlap

The original code does not prevent the following scenarios:
> sbi_ecall_register_extension(ext1); /* extension id (70-80) */
> sbi_ecall_register_extension(ext2); /* extension id (50-100) */

Signed-off-by: Xiang Wang <merle@hardenedlinux.org>
Reviewed-by: Anup Patel <anup.patel@wdc.com>