lib: sbi: Refine the settings for switching to Virtual Supervisor Mode.

Although Mstatus.MPV is set, before executing mret, access to VS mode
registers should use the actual register addresses, not the pseudonyms
of S registers.

Signed-off-by: Pope B.Lei <popeblei@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>