usb: ehci: remove old stub_debug_files definition

This patch removes the local STUB_DEBUG_FILES debugging
definition. STUB_DEBUG_FILES was used only in ehci-hcd, whereas
CONFIG_DYNAMIC_DEBUG is used all over the kernel.

Signed-off-by: Geyslan G. Bem <geyslan@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>