lib: Remove prints from sbi_hart_switch_mode()

The sbi_hart_switch_mode() will be used by CPU hotplug. This means
if we have prints in sbi_hart_switch_mode() then these prints will
mix with Linux prints.

Being a runtime firmware, we should be verbose only at coldboot
time and error situations.

Signed-off-by: Anup Patel <anup.patel@wdc.com>