lib: fix is_region_valid()

For 'reg->order == __riscv_xlen' the term 'BIT(reg->order)' is undefined.

Addresses-Coverity-ID: 1529706 ("Bad bit shift operation")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>