board: dragonboard410c: Fix button cmd name

board: dragonboard410c: Fix button cmd name

Commit 359e1d4a57e0 ("board: dragonboard410c: Use button_cmd instead of
custom code") was made in parallel with commit 8f5685d5d32f ("button:
qcom-pmic: prettify and standardise button labels"), which changed the
default button label from "vol_down" to "Volume Down". This is causing
errors in the console during boot now:

  No button labelled 'vol_down'

Fix this by using the new label.

Fixes: 359e1d4a57e0 ("board: dragonboard410c: Use button_cmd instead of custom code")
Fixes: 8f5685d5d32f ("button: qcom-pmic: prettify and standardise button labels")
Signed-off-by: Stephan Gerhold <stephan.gerhold@linaro.org>
Reviewed-by:
Reviewed-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250417-db410c-fixes2-v1-1-76ad994da152@linaro.org
Signed-off-by: Casey Connolly <casey.connolly@linaro.org>