coresight: etb10: adjust read pointer only when needed

The read pointer (read_ptr) needs to be adjusted only if its value
has gone beyond the length of the memory buffer.

Reported-by: Suzuki K Poulose <[e-mail]>
Signed-off-by: Mathieu Poirier <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/hwtracing/coresight/coresight-etb10.c

···
 		read_ptr = (write_ptr + drvdata->buffer_depth) -
 					to_read / ETB_FRAME_SIZE_WORDS;
 		/* Wrap around if need be*/
-		read_ptr &= ~(drvdata->buffer_depth - 1);
+		if (read_ptr > (drvdata->buffer_depth - 1))
+			read_ptr -= drvdata->buffer_depth;
 		/* let the decoder know we've skipped ahead */
 		local_inc(&buf->lost);
 	}