scmi_clock: Fix return code when resource is in requested state

The SCP firmware returned an error when an agent tries to set any
resource to its current state. This takes care of the behavior
when Uboot turns ON a resource and the kernel tries to again
enable it at boot (both are same agents on same mailbox channel).

Change-Id: Ib1f094ef008d36d9979f5b69a72440ca77c1813a
Signed-off-by: Leandro Belli <leandro.belli@arm.com>