IEEE 802.15.4: Fix an undefined behavior at runtime

The error was:
print-802_15_4.c:442:9: runtime error: implicit conversion from type
'int' of value 15840046 (32-bit, signed) to type 'uint16_t' (aka
'unsigned short') changed the value to 45870 (16-bit, unsigned)

Proposal of fix by Guy Harris:
"Those shifted values will be converted (in the C abstract machine) to
int, but they'll all be in the range 0 to 65535, as will be the XOR of
all 4 of them; converting that to a uint16_t isn't undefined behavior."

print-802_15_4.c

@@ -439,10 +439,10 @@ ieee802_15_4_crc16(const u_char *p,
 		/* Update CRC */
 		x = crc >> 8 ^ y;
 		x ^= x >> 4;
-		crc = (crc << 8) ^
-			((unsigned short)(x << 12)) ^
-			((unsigned short)(x <<5)) ^
-			((unsigned short)x);
+		crc = ((uint16_t)(crc << 8)) ^
+			((uint16_t)(x << 12)) ^
+			((uint16_t)(x << 5)) ^
+			((uint16_t)x);
 		data_len--;
 	}
 	/* Reverse bits on output */