alx: fix wrong condition to free descriptor memory

The condition to free the descriptor memory is wrong, we want to free the
memory if it is set and not if it is unset. Invert the test to fix this
issue.

Fixes: b0999223f224b ("alx: add ability to allocate and free alx_napi structures")
Signed-off-by: Tobias Regnery <tobias.regnery@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>