mmc: tmio: comment the ERR_PTR usage in this driver

It is not super obvious why the driver sometimes uses an ERR_PTR for the
current mrq. Explain why in comments.

Signed-off-by: Wolfram Sang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ulf Hansson <[e-mail]>

drivers/mmc/host/tmio_mmc_core.c

···
 		return;
 	}
 
+	/* Disallow new mrqs and work handlers to run */
 	host->mrq = ERR_PTR(-EBUSY);
 
 	spin_unlock_irqrestore(&host->lock, flags);
···
 			"%s.%d: IOS interrupted: clk %u, mode %u",
 			current->comm, task_pid_nr(current),
 			ios->clock, ios->power_mode);
-	host->mrq = NULL;
 
+	/* Ready for new mrqs */
+	host->mrq = NULL;
 	host->clk_cache = ios->clock;
 
 	mutex_unlock(&host->ios_lock);