net: phy: dp83640: Fixup cast to restricted __be16 warning

ntohs() expects to be passed a __be16. Correct the type of the
variable holding the sequence ID.

Cc: Richard Cochran <richardcochran@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>