RDMA: Fix return value check for ib_get_eth_speed()

ib_get_eth_speed() return 0 on success. Fixing the condition checking
and prevent reporting failure for query_port verb.

Fixes: d4186194 ("Add generic function to extract IB speed from netdev")
Signed-off-by: Selvin Xavier <selvin.xavier@broadcom.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>