thunderbolt: Fix a backport error for display flickering issue

A mistake was made when backporting commit 583893a ("thunderbolt: Fix
Thunderbolt 3 display flickering issue on 2nd hot plug onwards") in missing
the `if` block.  Add it back in.

Reported-by: Joakim.Tjernlund@infinera.com
Closes: https://lore.kernel.org/stable/28b5d0accce90bedf2f75d65290c5a1302225f0f.camel@infinera.com/
Fixes: 06614ca ("thunderbolt: Fix Thunderbolt 3 display flickering issue on 2nd hot plug onwards")
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/thunderbolt/tmu.c

@@ -416,6 +416,7 @@ int tb_switch_tmu_disable(struct tb_switch *sw)
 		 * mode.
 		 */
 		ret = tb_switch_tmu_rate_write(sw, TB_SWITCH_TMU_RATE_OFF);
+		if (ret)
 			return ret;
 
 		tb_port_tmu_time_sync_disable(up);