fw: rename GRXSTS[RP] constants to match OTG databook

Match the names used in the Synopsis OTG databook for consistency.

Signed-off-by: Paul Burton <paulburton89@gmail.com>
fw: only enable OUT EP when there's data to send

This seems to make things much more reliable. Remove the timing hack
that was somehow reducing the severity of the problem.

Signed-off-by: Paul Burton <paulburton89@gmail.com>
fw: warn on unhandled USB requests

Make it easier to debug what's missing.

Signed-off-by: Paul Burton <paulburton89@gmail.com>