net: dsa: sja1105: Fix bit offsets of index field from L2 lookup entries

This was inadvertently copied from the SJA1105 E/T structure and not
tested.  Cross-checking with the P/Q/R/S documentation (UM11040) makes
it immediately obvious what the correct bit offsets for this field are.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>