lib: sbi_hsm: Move misplaced comment

While non-retentive suspend is not allowed for M-mode, the comment
at the top of sbi_hsm_hart_suspend() implied suspend wasn't allowed
for M-mode at all. Move the comment above the mode check which is
inside a suspend type is non-retentive check.

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