net: dsa: bcm_sf2: Set correct CHAIN_ID and slice number mask

When configuring an IPv6 address mask, we should use SLICE_NUM_MASK as
the mask in order to make sure all bits are masked by the hardware.
Also, we want matching entries to have a CHAIN_ID value set to the same
value as the rule index we return to user-space for convenience, so fix
that too.

Fixes: ba0696c2 ("net: dsa: bcm_sf2: Add support for IPv6 CFP rules")
Fixes: dd8eff68

 ("net: dsa: bcm_sf2: Allow matching arbitrary IPv6 masks/lengths")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>