firmware: smci: possible NULL dereference

sandbox_scmi_devices_ctx() may return NULL. We should not dereference this
value in sandbox_scmi_devices_remove().

The problem was indicated by 'gcc-11 -fanalyzer'.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>