net/phy: Remove return value for void function

This was caught when using a spatch (aka. coccinelle) script
written by Joe Perches.

Cc: Joe Perches <joe@perches.com>
Signed-off-by: Shruti Kanetkar <Shruti@Freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>