usb: fix regression in SuperSpeed endpoint descriptor parsing

commit b37d83a6 ("usb: Parse the new USB 3.1 SuperSpeedPlus Isoc
endpoint companion descriptor") caused a regression in 4.6-rc1 and fails
to parse SuperSpeed endpoint companion descriptors.

The new SuperSpeedPlus Isoc endpoint companion parsing code incorrectly
decreased the the remaining buffer size before comparing the size with the
expected length of the descriptor.

This lead to possible failure in reading the SuperSpeed endpoint companion
descriptor of the last endpoint, displaying a message like:

"No SuperSpeed endpoint companion for config 1 interface 0 altsetting 0
 ep 129: using minimum values"

Fix it by decreasing the size after comparing it.
Also finish all the SS endpoint companion parsing before calling SSP isoc
endpoint parsing function.

Fixes: b37d83a6
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>