IMPROVED USAGE of STM32 USB RESOURCES

```
Description: The STM32 platforms use a non-standard, USB host peripheral
that uses "channels" to implement data transfers the current
logic associates each channel with an pipe/endpoint (with two
channels for bi-directional control endpoints). The OTGFS
peripheral has 8 channels and the OTGHS peripheral has 12
channels.

This works okay until you add a hub and try connect multiple
devices. A typical device will require 3-4 pipes and, hence,
4-5 channels. This effectively prevents using a hub with the
STM32 devices. This also applies to the EFM32 which uses the
same IP.

It should be possible to redesign the STM32 F4 OTGHS/OTGFS and
EFM32 host driver so that channels are dynamically assigned to
pipes as needed for individual transfers. Then you could have
more "apparent" pipes and make better use of channels.
Although there are only 8 or 12 channels, transfers are not
active all of the time on all channels so it ought to be
possible to have an unlimited number of "pipes" but with no
more than 8 or 12 active transfers.
Status: Open
Priority: Medium-Low
```