drm/amd/powerplay: Fix missing break in switch

Add missing break statement in order to prevent the code from falling
through to the default case.

The resoning for this is that pclk_vol_table is an automatic variable.
So, it makes no sense to update it just before falling through to the
default case and return -EINVAL.

This bug was found thanks to the ongoing efforts to enabling
-Wimplicit-fallthrough.

Fixes: cd70f3d6 ("drm/amd/powerplay: PP/DAL interface changes for dynamic clock switch")
Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>