ptp: ptp_clock: make scaled_ppm_to_ppb static inline

Make scaled_ppm_to_ppb static inline to be able to build drivers that
use this function even with PTP_1588_CLOCK disabled.

Signed-off-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@oss.nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>