usb: dwc3: clear 'res_trans_idx' as soon as it becomes invalid

Transfer resource index is cleared in hardware when XFERCOMPLETE
event is generated, so clear the driver's res_trans_idx variable
immediately after that event is received. The upcoming hibernation
patches depend on this change.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>