net: dsa: qca8k: fix non static symbol warning

Fixes the following sparse warning:

drivers/net/dsa/qca8k.c:259:22: warning:
 symbol 'qca8k_regmap_config' was not declared. Should it be static?

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