USB: OHCI: fix endless polling behavior

This patch (as1149) fixes an obscure problem in OHCI polling.  In the
current code, if the RHSC interrupt status flag turns on at a time
when RHSC interrupts are disabled, it will remain on forever:

	The interrupt handler is the only place where RHSC status
	gets turned back off;

	The interrupt handler won't turn RHSC status off because it
	doesn't turn off status flags if the corresponding interrupt
	isn't enabled;

	RHSC interrupts will never get enabled because
	ohci_root_hub_state_changes() doesn't reenable RHSC if RHSC
	status is on!

As a result we will continue polling indefinitely instead of reverting
to interrupt-driven operation, and the root hub will not autosuspend.
This particular sequence of events is not at all unusual; in fact
plugging a USB device into an OHCI controller will usually cause it to
occur.

Of course, this is a bug.  The proper thing to do is to turn off RHSC
status just before reading the actual port status values.  That way
either a port status change will be detected (if it occurs before the
status read) or it will turn RHSC back on.  Possibly both, but that
won't hurt anything.

We can still check for systems in which RHSC is totally broken, by
re-reading RHSC after clearing it and before reading the port
statuses.  (This re-read has to be done anyway, to post the earlier
write.)  If RHSC is on but no port-change statuses are set, then we
know that RHSC is broken and we can avoid re-enabling it.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>