net: sxgbe: fix sparse warnings about static declaration

This fixes followings:

sparse warnings: (new ones prefixed by >>)

>> drivers/net/ethernet/samsung/sxgbe/sxgbe_platform.c:197:5:
sparse: symbol 'sxgbe_platform_freeze' was not declared. Should it be static?
>> drivers/net/ethernet/samsung/sxgbe/sxgbe_platform.c:204:5:
sparse: symbol 'sxgbe_platform_restore' was not declared. Should it be static?
>> drivers/net/ethernet/samsung/sxgbe/sxgbe_platform.c:228:24:
sparse: symbol 'sxgbe_platform_driver' was not declared. Should it be static?

>> drivers/net/ethernet/samsung/sxgbe/sxgbe_main.c:1795:6:
sparse: symbol 'sxgbe_get_ops' was not declared. Should it be static?

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Byungho An <bh74.an@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>