Fix clearing of ISTAT in Kinetis USB

The ISTAT register is write 1 to clear. Because of this ORing this
register with itself fill clear all bits that are set. This patch
updates the code to use plain assignment so only desired
bit is cleared.

Author: c1728p9

features/unsupported/USBDevice/targets/TARGET_Freescale/USBHAL_KL25Z.cpp

@@ -438,7 +438,7 @@ void USBHAL::usbisr(void) {
     if (istat & 1<<7) {
         if (USB0->ENDPOINT[0].ENDPT & USB_ENDPT_EPSTALL_MASK)
             USB0->ENDPOINT[0].ENDPT &= ~USB_ENDPT_EPSTALL_MASK;
-        USB0->ISTAT |= USB_ISTAT_STALL_MASK;
+        USB0->ISTAT = USB_ISTAT_STALL_MASK;
     }
 
     // token interrupt
@@ -493,13 +493,13 @@ void USBHAL::usbisr(void) {
 
     // sleep interrupt
     if (istat & 1<<4) {
-        USB0->ISTAT |= USB_ISTAT_SLEEP_MASK;
+        USB0->ISTAT = USB_ISTAT_SLEEP_MASK;
     }
 
     // error interrupt
     if (istat & USB_ISTAT_ERROR_MASK) {
         USB0->ERRSTAT = 0xFF;
-        USB0->ISTAT |= USB_ISTAT_ERROR_MASK;
+        USB0->ISTAT = USB_ISTAT_ERROR_MASK;
     }
 }