Eliminate warnings (one of which looks like a bug)

arch/avr/src/at90usb/at90usb_usbdev.c

@@ -2229,7 +2229,7 @@ static int avr_epconfigure(FAR struct usbdev_ep_s *ep,
 
   /* Enable interrupts as appropriate for this endpoint */
 
-  UEIENX |= uecfg1x;
+  UEIENX |= ueienx;
   return OK;
 }
 

arch/avr/src/avr/up_spi.c

@@ -515,7 +515,9 @@ FAR struct spi_dev_s *up_spiinitialize(int port)
   /* Clear status flags by reading them */
 
   regval = SPSR;
+  UNUSED(regval);
   regval = SPDR;
+  UNUSED(regval);
 
   /* Set the initial SPI configuration */
 
@@ -538,4 +540,3 @@ FAR struct spi_dev_s *up_spiinitialize(int port)
   return &priv->spidev;
 }
 #endif /* CONFIG_AVR_SPI */
-