net: w5100: check return value after calling platform_get_resource()

[ Upstream commit a39ff4a47f3e1da3b036817ef436b1a9be10783a ]

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>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>