net: dsa: sja1105: Don't return a negative in u8 sja1105_stp_state_get

Dan Carpenter says:

The patch 640f763f: "net: dsa: sja1105: Add support for Spanning
Tree Protocol" from May 5, 2019, leads to the following static
checker warning:

        drivers/net/dsa/sja1105/sja1105_main.c:1073 sja1105_stp_state_get()
        warn: signedness bug returning '(-22)'

The caller doesn't check for negative errors anyway.

Fixes: 640f763f: ("net: dsa: sja1105: Add support for Spanning Tree Protocol")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>