net: dsa: lan9303: fix variable 'res' set but not used

Since lan9303_adjust_link() is a void function, there is no option to
return an error. So just remove the variable and lets any errors be
discarded.

Cc: Egil Hjelmeland <privat@egil-hjelmeland.no>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>