net: sparx5: fix compiletime_assert for GCC 4.9

Stephen reports sparx5 broke GCC 4.9 build.
Move the compiletime_assert() out of the static function.
Compile-tested only, no object code changes.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: f3cad261

 ("net: sparx5: add hostmode with phylink support")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>