[PATCH] USB: Code improvements for core/config.c

This patch makes some improvements to the code in config.c.

	Create a subroutine to handle the repeated task of skipping
	forward to the next descriptor of a certain type.

	Remove some fairly useless debugging messages (they could
	never even have been enabled in the pre-as221 code).

	Verify that endpoint descriptors don't have an address
	equal to 0 (as well as not being above 15).

	Rename some local variables so they are a little more
	consistent and meaningful.

Despite all the changes, the functionality should remain the same.
Please apply.