hw/qdev-clock: Avoid calling qdev_connect_clock_in after DeviceRealize

hw/qdev-clock: Avoid calling qdev_connect_clock_in after DeviceRealize

Clock canonical name is set in device_set_realized (see the block
added to hw/core/qdev.c in commit 0e6934f264).
If we connect a clock after the device is realized, this code is
not executed. This is currently not a problem as this name is only
used for trace events, however this disrupt tracing.

Add a comment to document qdev_connect_clock_in() must be called
before the device is realized, and assert this condition.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200803105647.22223-5-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>