pch_gbe: scale time stamps to nanoseconds

This patch fixes the helper functions that give the transmit and
receive time stamps to return nanoseconds, instead of arbitrary clock
ticks.

[ RC - Rebased Takahiro's changes and wrote a commit message
  explaining the changes. ]

Signed-off-by: Takahiro Shimizu <tshimizu818@gmail.com>
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>