w1:omap_hdq: fix regression

Commit e93762bbf681 ("w1: masters: omap_hdq: add support for 1-wire
mode") added a statement to clear the hdq_irqstatus flags in
hdq_read_byte().

If the hdq reading process is scheduled slowly or interrupts are
disabled for a while the hardware read activity might already be
finished on entry of hdq_read_byte().  And hdq_isr() already has set the
hdq_irqstatus to 0x6 (can be seen in debug mode) denoting that both, the
TXCOMPLETE and RXCOMPLETE interrupts occurred in parallel.

This means there is no need to wait and the hdq_read_byte() can just
read the byte from the hdq controller.

By resetting hdq_irqstatus to 0 the read process is forced to be always
waiting again (because the if statement always succeeds) but the
hardware will not issue another RXCOMPLETE interrupt.  This results in a
false timeout.

After such a situation the hdq bus hangs.

Link: http://lkml.kernel.org/r/b724765f87ad276a69625bc19806c8c8844c4590.1469513669.git.hns@goldelico.com
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Cc: Evgeniy Polyakov <zbr@ioremap.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/w1/masters/omap_hdq.c

@@ -390,8 +390,6 @@ static int hdq_read_byte(struct hdq_data *hdq_data, u8 *val)
 		goto out;
 	}
 
-	hdq_data->hdq_irqstatus = 0;
-
 	if (!(hdq_data->hdq_irqstatus & OMAP_HDQ_INT_STATUS_RXCOMPLETE)) {
 		hdq_reg_merge(hdq_data, OMAP_HDQ_CTRL_STATUS,
 			OMAP_HDQ_CTRL_STATUS_DIR | OMAP_HDQ_CTRL_STATUS_GO,