DM9000: Update retry count whilst identifying chip

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: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>