cpu/esp32: fixes esp_net maximum packet size

Fixes the maximum packet size of 255 bytes in the esp_eth netdev driver of ESP32 mcu.

fixup! cpu/esp32: fixes esp_net maximum packet size

cpu/esp32/esp-eth/esp_eth_netdev.c

@@ -243,7 +243,7 @@ static int _esp_eth_recv(netdev_t *netdev, void *buf, size_t len, void *info)
 
     mutex_lock(&dev->dev_lock);
 
-    uint8_t size = dev->rx_len;
+    int size = dev->rx_len;
 
     if (!buf && !len) {
         /* return the size without dropping received data */

cpu/esp32/esp-eth/esp_eth_netdev.h

@@ -37,11 +37,11 @@ typedef struct
 {
     netdev_t netdev;                    /**< netdev parent struct */
 
-    uint8_t rx_len;                     /**< number of bytes received */
-    uint8_t rx_buf[ETHERNET_DATA_LEN];  /**< receive buffer */
+    uint16_t rx_len;                     /**< number of bytes received */
+    uint16_t tx_len;                     /**< number of bytes in transmit buffer */
 
-    uint8_t tx_len;                     /**< number of bytes in transmit buffer */
-    uint8_t tx_buf[ETHERNET_DATA_LEN];  /**< transmit buffer */
+    uint8_t  rx_buf[ETHERNET_DATA_LEN];  /**< receive buffer */
+    uint8_t  tx_buf[ETHERNET_DATA_LEN];  /**< transmit buffer */
 
     uint32_t event;                     /**< received event */
     bool     link_up;                   /**< indicates whether link is up */