firmware: arm_scmi: Fix base agent discover response

firmware: arm_scmi: Fix base agent discover response

According to scmi specification, the response of the discover agent request
is made of:
- int32 status
- uint32 agent_id
- uint8 name[16]

but the current implementation doesn't take into account the agent_id field
and only allocates a rx buffer of SCMI_MAX_STR_SIZE length

Allocate the correct length for rx buffer and copy the name from the
correct offset in the response.

While no error were returned until v5.15, v5.16-rc1 fails with virtio_scmi
transport channel:

 | arm-scmi firmware:scmi0: SCMI Notifications - Core Enabled.
 | arm-scmi firmware:scmi0: SCMI Protocol v2.0 'Linaro:PMWG' Firmware version 0x2090000
 | scmi-virtio virtio0: tx:used len 28 is larger than in buflen 24

Link: https://lore.kernel.org/r/20211117081856.9932-1-vincent.guittot@linaro.org
Fixes: b6f20ff8bd94 ("firmware: arm_scmi: add common infrastructure and support for base protocol")
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>