net: dsa: tag_ksz: Fix __be16 warnings

cpu_to_be16 returns a __be16 value. So what it is assigned to needs to
have the same type to avoid warnings.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>