The function dma_alloc_coherent() in qed_vf_hw_prepare() can fail, so
its return value should be checked.

Fixes: 1408cc1f ("qed: Introduce VFs")
Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>