Reading the ID register does not always return the correct ID
from the device, so we retry several times to see if we get
a correct value.

These failures seem to be excaserbated by the speed of the
access to the chip (possibly time between issuing the address
and then the data cycle).
Signed-off-by: NBen Dooks <ben-linux@fluff.org>
Signed-off-by: NJeff Garzik <jeff@garzik.org>