usb/hcd: asserts should not have side effects

They can be deactivated with -DNDEBUG, which is why they can't contain
code which should always be run.

It seems the local implementation of USB_ASSERT doesn't honor the
CPP flags NDEBUG, this will be corrected in a later patch.

Change-Id: Iac56b09fd563f1b8c4c7be15a468a05b9cc86a18