net: dsa: tag_sja1105: appease sparse checks for ethertype accessors

A comparison between a value from the packet and an integer constant
value needs to be done by converting the value from the packet from
net->host, or the constant from host->net. Not the other way around.
Even though it makes no practical difference, correct that.

Fixes: 38b5beea

 ("net: dsa: sja1105: prepare tagger for handling DSA tags and VLAN simultaneously")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>