hwrng: omap - Only fail if pm_runtime_get_sync returns < 0

Currently omap-rng checks the return value of pm_runtime_get_sync and
reports failure if anything is returned, however it should be checking
if ret < 0 as pm_runtime_get_sync return 0 on success but also can return
1 if the device was already active which is not a failure case. Only
values < 0 are actual failures.

Fixes: 61dc0a44

 ("hwrng: omap - Fix assumption that runtime_get_sync will always succeed")
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>