net: dsa: sja1105: Plug in support for TCAM searches via the dynamic interface

Only a single dynamic configuration table of the SJA1105 P/Q/R/S
supports this operation: the FDB.

To keep the existing structure in place (sja1105_dynamic_config_read and
sja1105_dynamic_config_write) and not introduce any new function, a
convention is made for sja1105_dynamic_config_read that a negative index
argument denotes a search for the entry provided as argument.

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