net: dsa: sja1105: Do not use address of compatible member in sja1105_check_device_id

Clang warns:

drivers/net/dsa/sja1105/sja1105_main.c:3418:38: warning: address of
array 'match->compatible' will always evaluate to 'true'
[-Wpointer-bool-conversion]
        for (match = sja1105_dt_ids; match->compatible; match++) {
        ~~~                          ~~~~~~~^~~~~~~~~~
1 warning generated.

We should check the value of the first character in compatible to see if
it is empty or not. This matches how the rest of the tree iterates over
IDs.

Fixes: 0b0e2997 ("net: dsa: sja1105: use detected device id instead of DT one on mismatch")
Link: https://github.com/ClangBuiltLinux/linux/issues/1139


Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>