Fix Tx FIFO memory calculation

This used to allocate 9 * 16 words minimum, no matter how many tx
endpoints where active. Now we only account for memory allocated to
enabled Tx endpoints when determining how much has already been used.

Author: dgoodlad

src/endpoint_memory.rs

@@ -152,9 +152,11 @@ impl<USB: UsbPeripheral> EndpointMemoryAllocator<USB> {
             return Err(UsbError::InvalidEndpoint)
         }
 
-        let used = 30
-            + self.total_rx_buffer_size_words() as usize
-            + self.tx_fifo_size_words.iter().sum::<u16>() as usize;
+        let mut used = self.total_rx_buffer_size_words() as usize + 30;
+        for sz in &self.tx_fifo_size_words[0..ep_number] {
+            used += core::cmp::max(*sz as usize, 16);
+        }
+        used -= 16;
 
         let size_words = core::cmp::max((size + 3) / 4, 16);
         if (used + size_words) > USB::FIFO_DEPTH_WORDS {