ethernet: ldmvsw: mark ldmvsw_open() static

The function is exported for no reason and should just be static:

drivers/net/ethernet/sun/ldmvsw.c:127:5: error: no previous prototype for 'ldmvsw_open' [-Werror=missing-prototypes]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org> # build-tested
Link: https://lore.kernel.org/r/20230810122528.1220434-1-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>