drm/panel: boe-tv101wum-nl6: Check for errors on the NOP in prepare()

The mipi_dsi_dcs_nop() function returns an error but we weren't
checking it in boe_panel_prepare(). Add a check. This is highly
unlikely to matter in practice. If the NOP failed then likely later
MIPI commands would fail too.

Found by code inspection.

Fixes: 812562b8d881 ("drm/panel: boe-tv101wum-nl6: Fine tune the panel power sequence")
Signed-off-by: Douglas Anderson <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/20240517143643.3.Ibffbaa5b4999ac0e55f43bf353144433b099d727@changeid
Signed-off-by: Neil Armstrong <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517143643.3.Ibffbaa5b4999ac0e55f43bf353144433b099d727@changeid

drivers/gpu/drm/panel/panel-boe-tv101wum-nl6.c

···
 	usleep_range(10000, 11000);
 
 	if (boe->desc->lp11_before_reset) {
-		mipi_dsi_dcs_nop(boe->dsi);
+		ret = mipi_dsi_dcs_nop(boe->dsi);
+		if (ret < 0) {
+			dev_err(&boe->dsi->dev, "Failed to send NOP: %d\n", ret);
+			goto poweroff;
+		}
 		usleep_range(1000, 2000);
 	}
 	gpiod_set_value(boe->enable_gpio, 1);