net: phy: mdio-bcm-unimac: fix unsigned wrap-around when decrementing timeout

Change post-decrement compare to pre-decrement to avoid an
unsigned integer wrap-around on timeout. This leads to the following
!timeout check to never to be true so -ETIMEDOUT is never returned.

Detected by CoverityScan, CID#1452623 ("Logically dead code")

Fixes: 69a60b05 ("net: phy: mdio-bcm-unimac: factor busy polling loop")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>