firmware: arm_scmi: Avoid double free in error flow

If device_register() fails, both put_device() and kfree() are called,
ending with a double free of the scmi_dev.

Calling kfree() is needed only when a failure happens between the
allocation of the scmi_dev and its registration, so move it to there
and remove it from the error flow.

Fixes: 46edb8d1 ("firmware: arm_scmi: provide the mandatory device release callback")
Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>