samd: Fix a lock-up situation at high traffic.

This PR fixes a transmit lock-up, which happens, when data is received
and sent at the sime time at moderate to high speeds, like code
which just echoes incoming data.

In my case, an issue was reported here:
micropython/micropython#8521

Author: robert-hh

src/portable/microchip/samd/dcd_samd.c

@@ -286,14 +286,14 @@ bool dcd_edpt_xfer (uint8_t rhport, uint8_t ep_addr, uint8_t * buffer, uint16_t
   {
     bank->PCKSIZE.bit.MULTI_PACKET_SIZE = total_bytes;
     bank->PCKSIZE.bit.BYTE_COUNT = 0;
-    ep->EPSTATUSCLR.reg |= USB_DEVICE_EPSTATUSCLR_BK0RDY;
-    ep->EPINTFLAG.reg |= USB_DEVICE_EPINTFLAG_TRFAIL0;
+    ep->EPSTATUSCLR.reg = USB_DEVICE_EPSTATUSCLR_BK0RDY;
+    ep->EPINTFLAG.reg = USB_DEVICE_EPINTFLAG_TRFAIL0;
   } else
   {
     bank->PCKSIZE.bit.MULTI_PACKET_SIZE = 0;
     bank->PCKSIZE.bit.BYTE_COUNT = total_bytes;
-    ep->EPSTATUSSET.reg |= USB_DEVICE_EPSTATUSSET_BK1RDY;
-    ep->EPINTFLAG.reg |= USB_DEVICE_EPINTFLAG_TRFAIL1;
+    ep->EPSTATUSSET.reg = USB_DEVICE_EPSTATUSSET_BK1RDY;
+    ep->EPINTFLAG.reg = USB_DEVICE_EPINTFLAG_TRFAIL1;
   }
 
   return true;