USB: EHCI: Rearrange EHCI_URB_TRACE code to avoid GCC-4.6 warnings.

With pre-release GCC-4.6, we get a 'set but not used' warning when
EHCI_URB_TRACE is not set because we set the qtd variable without
using it.

Rearrange the statements so that we only set qtd if it will be used.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>