lib: sbi_hart: Shorten the code to set MPV bit

MPV bit is set when the value of next_virt boolean variable equals
true. Since the value of next_virt is either 0 or 1, we can set
MPV bit without if-else logic.

Signed-off-by: Che-Chia Chang <alvinga@andestech.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>