Set USB SRAM QoS to sensitive latency

Author: sameer

cores/arduino/USB/SAMD21_USBDevice.h

@@ -61,6 +61,10 @@ class USBDevice_SAMD21G18x {
 	inline void noRunInStandby() { usb.CTRLA.bit.RUNSTDBY = 0; }
 	inline void wakeupHost()     { usb.CTRLB.bit.UPRSM = 1; }
 
+	// USB QoS
+	inline void setDataSensitiveQoS() { usb.QOSCTRL.bit.DQOS = 2; }
+	inline void setConfigSensitiveQoS() { usb.QOSCTRL.bit.CQOS = 2; }
+
 	// USB speed
 	inline void setFullSpeed()       { usb.CTRLB.bit.SPDCONF = USB_DEVICE_CTRLB_SPDCONF_FS_Val;   }
 	inline void setLowSpeed()        { usb.CTRLB.bit.SPDCONF = USB_DEVICE_CTRLB_SPDCONF_LS_Val;   }

cores/arduino/USB/USBCore.cpp

@@ -370,6 +370,8 @@ void USBDeviceClass::init()
 	usbd.reset();
 
 	usbd.calibrate();
+	usbd.setDataSensitiveQoS();
+	usbd.setConfigSensitiveQoS();
 	usbd.setUSBDeviceMode();
 	usbd.runInStandby();
 	usbd.setFullSpeed();