usb: stm32: clarify calculation of FIFO sizes

Magic constants throughout the code made this difficult to reason about,
especially with two different units of measurement (bytes and words) at
play.

Signed-off-by: Armin Brauns <armin.brauns@embedded-solutions.at>

drivers/usb/device/usb_dc_stm32.c

@@ -130,14 +130,14 @@ static const struct gpio_dt_spec ulpi_reset =
 /* We need n TX IN FIFOs */
 #define TX_FIFO_NUM USB_NUM_BIDIR_ENDPOINTS
 
-/* We need a minimum size for RX FIFO */
+/* We need a minimum size for RX FIFO - exact number seemingly determined through trial and error */
 #define RX_FIFO_EP_WORDS 160
 
-/* 4-byte words TX FIFO */
-#define TX_FIFO_WORDS ((USB_RAM_SIZE - 64) / 4 - RX_FIFO_EP_WORDS)
-
 /* Allocate FIFO memory evenly between the TX FIFOs */
 /* except the first TX endpoint need only 64 bytes */
+#define TX_FIFO_EP_0_WORDS 16
+#define TX_FIFO_WORDS (USB_RAM_SIZE / 4 - RX_FIFO_EP_WORDS - TX_FIFO_EP_0_WORDS)
+/* Number of words for each remaining TX endpoint FIFO */
 #define TX_FIFO_EP_WORDS (TX_FIFO_WORDS / (TX_FIFO_NUM - 1))
 
 #endif /* USB */
@@ -450,7 +450,8 @@ static int usb_dc_stm32_init(void)
 	for (i = 0U; i < USB_NUM_BIDIR_ENDPOINTS; i++) {
 		if (i == 0) {
 			/* first endpoint need only 64 byte for EP_TYPE_CTRL */
-			HAL_PCDEx_SetTxFiFo(&usb_dc_stm32_state.pcd, i,	16);
+			HAL_PCDEx_SetTxFiFo(&usb_dc_stm32_state.pcd, i,
+					TX_FIFO_EP_0_WORDS);
 		} else {
 			HAL_PCDEx_SetTxFiFo(&usb_dc_stm32_state.pcd, i,
 					TX_FIFO_EP_WORDS);