USB: isp1760: Implement solution for erratum 2

USB: isp1760: Implement solution for erratum 2

The document says:
|2.1 Problem description
|    When at least two USB devices are simultaneously running, it is observed that
|    sometimes the INT corresponding to one of the USB devices stops occurring. This may
|    be observed sometimes with USB-to-serial or USB-to-network devices.
|    The problem is not noticed when only USB mass storage devices are running.
|2.2 Implication
|    This issue is because of the clearing of the respective Done Map bit on reading the ATL
|    PTD Done Map register when an INT is generated by another PTD completion, but is not
|    found set on that read access. In this situation, the respective Done Map bit will remain
|    reset and no further INT will be asserted so the data transfer corresponding to that USB
|    device will stop.
|2.3 Workaround
|    An SOF INT can be used instead of an ATL INT with polling on Done bits. A time-out can
|    be implemented and if a certain Done bit is never set, verification of the PTD completion
|    can be done by reading PTD contents (valid bit).
|    This is a proven workaround implemented in software.

Russell King run into this with an USB-to-serial converter. This patch
implements his suggestion to enable the high frequent SOF interrupt only
at the time we have ATL packages queued. It goes even one step further
and enables the SOF interrupt only if we have more than one ATL packet
queued at the same time.

Cc: <stable@kernel.org> # [2.6.35.x, 2.6.36.x, 2.6.37.x]
Tested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>