Fix sparse warning:

drivers/phy/ti/phy-am654-serdes.c:250:12: warning:
 symbol 'serdes_am654_xlate' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>