firmware: arm_scmi: Rework scmi_sensors_protocol_init

Properly handle return values from initialization helpers and avoid
setting sensor_ops before sensor_priv.

Link: https://lore.kernel.org/r/20201119174906.43862-2-cristian.marussi@arm.com


Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>