drm/imagination: Fix IS_ERR() vs NULL bug in pvr_request_firmware()

The pvr_build_firmware_filename() function returns NULL on error.  It
doesn't return error pointers.

Fixes: f99f5f3e ("drm/imagination: Add GPU ID parsing and firmware loading")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/384288de-a779-46c7-869d-b3c63462e12b@moroto.mountain

drivers/gpu/drm/imagination/pvr_device.c

@@ -286,8 +286,8 @@ pvr_request_firmware(struct pvr_device *pvr_dev)
 
 	filename = pvr_build_firmware_filename(pvr_dev, "powervr/rogue",
 					       PVR_FW_VERSION_MAJOR);
-	if (IS_ERR(filename))
-		return PTR_ERR(filename);
+	if (!filename)
+		return -ENOMEM;
 
 	/*
 	 * This function takes a copy of &filename, meaning we can free our