net: dsa: vsc73xx: Make some functions static

Fixes the following sparse warnings:

drivers/net/dsa/vitesse-vsc73xx.c:1054:6: warning:
 symbol 'vsc73xx_get_strings' was not declared. Should it be static?
drivers/net/dsa/vitesse-vsc73xx.c:1113:5: warning:
 symbol 'vsc73xx_get_sset_count' was not declared. Should it be static?
drivers/net/dsa/vitesse-vsc73xx.c:1122:6: warning:
 symbol 'vsc73xx_get_ethtool_stats' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>