powerpc: bpf: Fix the broken LD_VLAN_TAG_PRESENT test

We have to return the boolean here if the tag presents
or not, not just ANDing the TCI with the mask which results to:

[  709.412097] test_bpf: #18 LD_VLAN_TAG_PRESENT
[  709.412245] ret 4096 != 1
[  709.412332] ret 4096 != 1
[  709.412333] FAIL (2 times)

Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
Signed-off-by: David S. Miller <davem@davemloft.net>