remoteproc: k3-dsp: Use symbolic error codes in error messages

The error message

	failed to send mailbox message (-EINVAL)

is (for a human) more useful than

	failed to send mailbox message, status = -22

Adapt all error messages to use the symbolic names instead of the
numeric constants. The error paths in .probe() make use of
dev_err_probe() which automatically handles EPROBE_DEFER.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20231123211657.518181-7-u.kleine-koenig@pengutronix.de>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>