usb: fix loop in usb_config_parse

By inspection, index increment was missing.

PR: 281843
Reported by: Matt Jacobson
Reviewed by: bz, markj
Fixes: e4611d26265f ("usb(4): Call optional endpoint_uninit() when changing configuration or alternate setting.")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 114080d19973331426cd826f3a961c6ea9216a53)