smsc75xx: add explicit test that device is READY

This patch adds an explicit test that the READY bit is set on
the device when attempting to initialize it.

If this bit is clear then the device hasn't succesfully started
all its clocks, and this patch helps make the resulting logged
error more helpful.

Signed-off-by: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: David S. Miller <davem@davemloft.net>