taprio: make clock reference conversions easier

Later in this series we will need to transform from
CLOCK_MONOTONIC (used in TCP) to the clock reference used in TAPRIO.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Vedang Patel <vedang.patel@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>