st_kim: allow suspend if callback is not registered

Suspend/resume was failing if callbacks were not registered.
As it is ok not to do anything when suspending fix this
so it soen't return an error and allow the system to suspend.

Signed-off-by: Eyal Reizer <eyalr@ti.com>
Signed-off-by: Gigi Joseph <gigi.joseph@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>