rocker: mark parameters and local variables as const

Mark parameters and local variables as const where possible.
Signed-off-by: NSimon Horman <simon.horman@netronome.com>
Acked-by: NScott Feldman <sfeldma@gmail.com>
Acked-by: NJiri Pirko <jiri@resnulli.us>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>