dp83640: reduce driver noise

The driver has two warning messages that might be triggered
by normal use cases. When they appear, the messages give the
impression of a never ending series of errors.

This commit changes them to debug messages instead.

Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>