USB: Fix unused label warnings in drivers/usb/host/ehci-hcd.c

This gets rid of an annoying warning in ehci-hcd.c when DEBUG isn't
enabled:

    warning: label 'err_debug' defined but not used

by moving it inside the already-existing #ifdef DEBUG, so that it
matches the goto.  And now my regular build is warning-free again.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>