EthernetUdp (lm4f) - Some packets don't get properly received

I found a bug in /hardware/lm4f/libraries/Ethernet/EthernetUdp.cpp
Packets which contain 0xFF bytes will be truncated on that position when calling Udp.read(...)
To fix this I changed the following line in 

```C
read(unsigned char* buffer, size_t len)
```

from:
```C
int8_t b;
```

to this:
```C
int b;
```

Packets which contain 0xFF (-1) will be truncated because they are wrongly interpreted as error code.