USB: EHCI: compute full-speed bandwidth usage correctly

Although the bandwidth statistics maintained by ehci-hcd show up only
in the /sys/kernel/debug/usb/devices file, they ought to be calculated
correctly.  The calculation for full-speed isochronous endpoints is
wrong; it mistakenly yields bytes per microframe instead of bytes per
frame.  The "interval" value, which is in frames, should not be
converted to microframes.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>