Quick fix for ISO endpoints

[kiffie]

This is a quick fix for isochronous (ISO) endpoints that does not require changes on the usb-device.

There is already PR #29 for ISO endpoints pending, which seems to aim at a more sophisticated solution. I propose to integrate the present PR as an intermediate solution.

Successfully tested on STM32F446 on FS using a Linux host with different values for endpoint bInterval. Got reports on successful experiments on HS.

Primary use case: audio streaming according to USB Audio Device Class, Release 1.0