lib: sbi: Fix is_region_valid()

When order is equal to __riscv_xlen, the shift operation will not perform
any operation, which will cause reg->base & (BIT(reg->order) - 1) to always
be 0, and the condition has not been established.

This patch fixes this bug.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>