net/smsc911x: Always wait for the chip to be ready

Wait for the chip to be ready before any access to it. On the
Snowball platform we need to enable an external regulator before
the chip comes online, and then it happens that the device is
not yet ready at probe time, so let's wait for it.

Signed-off-by: Robert Marklund <robert.marklund@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>