net: dsa: sja1105: suppress -Wmissing-prototypes in sja1105_static_config.c

Newer compilers complain with W=1 builds that there are non-static
functions defined in sja1105_static_config.c that don't have a
prototype, because their prototype is defined in sja1105.h which this
translation unit does not include.

I don't entirely understand what is the point of these warnings, since
in principle there's nothing wrong with that. But let's move the
prototypes to a header file that _is_ included by
sja1105_static_config.c, since that will make these warnings go away.

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>