nfp: return nfp_rtsym_read_le() errors correctly

nfp_rtsym_read_le() has an out parameter for error codes.
We have to use that instead of returning errors directly.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>