net: dsa: sja1105: constify the sja1105_regs structures

The struct sja1105_regs tables are not modified during the runtime of
the driver, so they can be made constant. In fact, struct sja1105_info
already holds a const pointer to these.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>