net: dsa: tag_mtk: Fix warnings for __be16

net/dsa/tag_mtk.c:84:13: warning: incorrect type in assignment (different base types)
net/dsa/tag_mtk.c:84:13:    expected restricted __be16 [usertype] hdr
net/dsa/tag_mtk.c:84:13:    got int
net/dsa/tag_mtk.c:94:17: warning: restricted __be16 degrades to integer

The result of a ntohs() is not __be16, but u16.

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>