i2c: testunit: avoid re-issued work after read message

The to-be-fixed commit rightfully prevented that the registers will be
cleared. However, the index must be cleared. Otherwise a read message
will re-issue the last work. Fix it and add a comment describing the
situation.

Fixes: c422b6a63024 ("i2c: testunit: don't erase registers after STOP")
Signed-off-by: Wolfram Sang <[e-mail]>
Reviewed-by: Andi Shyti <[e-mail]>
Signed-off-by: Wolfram Sang <[e-mail]>

drivers/i2c/i2c-slave-testunit.c

···
 			queue_delayed_work(system_long_wq, &tu->worker,
 					   msecs_to_jiffies(10 * tu->regs[TU_REG_DELAY]));
 		}
+
+		/*
+		 * Reset reg_idx to avoid that work gets queued again in case of
+		 * STOP after a following read message. But do not clear TU regs
+		 * here because we still need them in the workqueue!
+		 */
+		tu->reg_idx = 0;
 		break;
 
 	case I2C_SLAVE_WRITE_REQUESTED: