net: Use 0.5 sec timeout in miiphy_reset() instead of counting loop

This patch fixes a problem I've notived on a buggy PPC4xx system. This
system has problems with the PHY MDIO communication and seemed to be
stuck/crashed in miiphy_reset(). But degugging revealed, that the CPU
didn't crash, but "only" hung in this counting loop for about 2 minutes.

This patch now uses a real timeout of 0.5 seconds (as mentioned in the
comment in miiphy_reset).

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>