Prevent Kinetis USB setup packets at wrong time

When disconnecting USB clear all pending interrupts. This ensures that
pending setup packets are dropped rather than being triggered after
disconnect.

Author: c1728p9

usb/device/targets/TARGET_Freescale/USBPhy_Kinetis.cpp

@@ -203,6 +203,10 @@ void USBPhyHw::disconnect()
     USB0->CTL &= ~USB_CTL_USBENSOFEN_MASK;
     // Pull up disable
     USB0->CONTROL &= ~USB_CONTROL_DPPULLUPNONOTG_MASK;
+
+    while (USB0->ISTAT) {
+        USB0->ISTAT = 0xFF;
+    }
 }
 
 void USBPhyHw::configure()