dp83640: Fix length check for event timestamp status messages

Event timestamp status messages have a variable length, ranging from
1 to 5 words (16 bit words). The current code however requires
a minimum message length of sizeof(*phy_txts). In most cases this
condition is fulfilled due to padding bytes. However, if several events
are signaled in a single message, padding bytes may not be present.
For short event timestamp status messages, the length check will fail,
and the event timestamp will be dropped.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>