pch_gbe: remove suspicious comment

The time stamping code in this driver appears to have been copied from
the ixp4xx_eth.c driver, including this timing comment. I had actually
measured the time stamp delay on an IXP425, but I really doubt that this
value also applies here.

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