sfc/siena: fix null pointer dereference in efx_hard_start_xmit

sfc/siena: fix null pointer dereference in efx_hard_start_xmit

[ Upstream commit 589c6eded10c77a12b7b2cf235b6b19a2bdb91fa ]

Like in previous patch for sfc, prevent potential (but unlikely) NULL
pointer dereference.

Fixes: 12804793b17c ("sfc: decouple TXQ type from label")
Reported-by: Tianhao Zhao <tizhao@redhat.com>
Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Link: https://lore.kernel.org/r/20220915141958.16458-1-ihuguet@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>