net: phy: fix less than zero comparison with unsigned variable val

The unsigned variable val is being checked for an error by checking
if it is less than zero. This can never occur because val is unsigned.
Fix this by making val a plain int.

Addresses-Coverity: ("Unsigned compared against zero")
Fixes: bdbdac76

 ("ethtool: provide UAPI for PHY master/slave configuration.")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>