net: phy: fix sign type error in genphy_config_eee_advert

In genphy_config_eee_advert, the return value of phy_read_mmd_indirect is
checked to know if the register could be accessed but the result is
assigned to a 'u32'.
Changing to 'int' to correctly get errors from phy_read_mmd_indirect.

Fixes: d853d145

 ("net: phy: add an option to disable EEE advertisement")
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>