firmware: arm_scmi: Drop empty stub for smc_mark_txdone

The scmi protocol core driver check for non NULL mark_txdone before
invoking the same. There is no need to provide a empty stub. SMC/HVC
calls are synchronous and the call return indicates the completion.

Link: https://lore.kernel.org/r/20200327163654.13389-3-sudeep.holla@arm.com


Tested-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>