firmware: arm_scmi: Fix double free in OPTEE transport

Channels can be shared between protocols, avoid freeing the same channel
descriptors twice when unloading the stack.

Fixes: 5f90f189 ("firmware: arm_scmi: Add optee transport")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Peng Fan <peng.fan@nxp.com>  #i.MX95 19x19 EVK
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
Message-Id: <20240812173340.3912830-2-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>