USB: Check on the number of bytes to send to not send more than expected

[sylvainpelissier]

Previously if _cmark was less than _cend but the length to send is more than _cend - _cmark then the device will send more data than expected and may crash the host driver.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}