dp83640: trivial fixes

This patch cleans up the input checking code on the external time stamp
function by using an unsigned rather than a signed channel index.

Also, this patch corrects the author's email address. When this macro
was last changed, the top level domain part of the email address was
left behind.

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