mdio: mux: fix an incorrect less than zero error check using a u32

The u32 variable v is being checked to see if an error return is
less than zero and this check has no effect because it is unsigned.
Fix this by making v and int (this also matches the type of
cb->bus_number which is assigned to the value in v).

Detected by CoverityScan, CID#1440454 ("Unsigned compared against zero")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>