liquidio: Fix an issue with multiple switchdev enable disables

Return success if the same dispatch function is being registered for
a given opcode and subcode, there by allow multiple switchdev enable
and disables.

Signed-off-by: Vijaya Mohan Guvva <vijaya.guvva@cavium.com>
Signed-off-by: Satanand Burla <satananda.burla@cavium.com>
Signed-off-by: Felix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>