lib: sbi: Don't handle VS-mode ecall in sbi_trap_handler()

The VS-mode ecall is supposed to be handled by HS-mode so
sbi_trap_handler() should fallback to default case for
VS-mode ecall trap.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>