HPS USB stack API change

[Hans Petter Selasky]

Hi all!

I'm planning to add another argument to "usbd_start_hardware()", which is a 
delay value that gives the delay in microseconds or maybe milliseconds before 
the transfer is actually queued. This can be used to reduce the polling rate 
on BULK-IN transfers among other things, to reduce the overall power usage, 
and to safely create infinite polling loops without fearing BUS overusage in 
case of failure conditions.

For example:

usbd_start_hardware(xfer, 200000); //200 ms delay

A delay value of zero indicates no delay.

I plan to implement this re-using the timeout callout.

Any comments ?

--HPS