phy: make phy_set_max_speed() *void*

After following the call tree of phy_set_max_speed(), it became clear
that this function never returns anything but 0, so we can change its
result type to *void* and drop the result checks from the three drivers
that actually bothered to do it...

Found by Linux Verification Center (linuxtesting.org) with the SVACE static
analysis tool.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>