hwrng: amd - Fix return value check in mod_init()

In case of error, the function devm_kzalloc() or devm_ioport_map()
return NULL pointer not ERR_PTR(). The IS_ERR() test in the return
value check should be replaced with NULL test.

Fixes: 31b2a73c ("hwrng: amd - Migrate to managed API")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>