net_sched: fix use of uninitialized ethertype variable in cls_flower

The addition of VLAN support caused a possible use of uninitialized
data if we encounter a zero TCA_FLOWER_KEY_ETH_TYPE key, as pointed
out by "gcc -Wmaybe-uninitialized":

net/sched/cls_flower.c: In function 'fl_change':
net/sched/cls_flower.c:366:22: error: 'ethertype' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This changes the code to only set the ethertype field if it
was nonzero, as before the patch.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 9399ae9a

 ("net_sched: flower: Add vlan support")
Cc: Hadar Hen Zion <hadarh@mellanox.com>
Cc: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>