qed: Fix potential use-after-free in qed_spq_post()

We need to check if p_ent->comp_mode is QED_SPQ_MODE_EBLOCK before
calling qed_spq_add_entry().  The test is fine is the mode is EBLOCK,
but if it isn't then qed_spq_add_entry() might kfree(p_ent).

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: David S. Miller <davem@davemloft.net>