usb: ehci: Reduce overhead of the scan_periodic loop

usb: ehci: Reduce overhead of the scan_periodic loop

scan_periodic is called with irq disabled. Merged Alan Stern's patch
to reduce the overhead of scan_periodic:

http://article.gmane.org/gmane.linux.usb.general/37441
Here is a patch which ought to reduce the overhead of the loop. It
avoids doing the expensive call to qh_completions() more than once
for each qh.

Change-Id: I218c75a1ce21edd3d58c7e8abd3e7f75880b6ad0
Signed-off-by: Benoit Goby <benoit@android.com>