phy: Sparx5 Eth SerDes: check return value after calling platform_get_resource()

It will cause null-ptr-deref if platform_get_resource() returns NULL,
we need check the return value.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20210603051014.2674744-1-yangyingliang@huawei.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>