net: netsec: remove superfluous if statement

While freeing tx buffers the memory has to be unmapped if the packet was
an skb or was used for .ndo_xdp_xmit using the same arguments. Get rid
of the unneeded extra 'else if' statement

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>