mctp: serial: Fix starting value for frame check sequence

RFC1662 defines the start state for the crc16 FCS to be 0xffff, but
we're currently starting at zero.

This change uses the correct start state. We're only early in the
adoption for the serial binding, so there aren't yet any other users to
interface to.

Fixes: a0c2ccd9b5ad ("mctp: Add MCTP-over-serial transport binding")
Reported-by: Harsh Tyagi <[e-mail]>
Tested-by: Harsh Tyagi <[e-mail]>
Signed-off-by: Jeremy Kerr <[e-mail]>
Reviewed-by: Alexander Duyck <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/mctp/mctp-serial.c

···
 #define BYTE_FRAME		0x7e
 #define BYTE_ESC		0x7d
 
+#define FCS_INIT		0xffff
+
 static DEFINE_IDA(mctp_serial_ida);
 
 enum mctp_serial_state {
···
 		buf[2] = dev->txlen;
 
 		if (!dev->txpos)
-			dev->txfcs = crc_ccitt(0, buf + 1, 2);
+			dev->txfcs = crc_ccitt(FCS_INIT, buf + 1, 2);
 
 		txlen = write_chunk(dev, buf + dev->txpos, 3 - dev->txpos);
 		if (txlen <= 0) {
···
 	case 1:
 		if (c == MCTP_SERIAL_VERSION) {
 			dev->rxpos++;
-			dev->rxfcs = crc_ccitt_byte(0, c);
+			dev->rxfcs = crc_ccitt_byte(FCS_INIT, c);
 		} else {
 			dev->rxstate = STATE_ERR;
 		}