bt: use the maximum retry count returned by the BMC
bt: use the maximum retry count returned by the BMC

OpenPower systems using a AMI firmware on the BMC have a BT device
configured with a capability of '1' maximum retry. The following code
is equivalent to what skiboot currently supports but it will now also
handle setups of other BT devices, like in qemu or OpenBMC.

Signed-off-by: Cédric Le Goater <2d8cace2254d4e43578d21933db015d84bcc468a@fr.ibm.com>
Reviewed-by: Vipin K Parashar <f7dae5427b3dec859a8db9747c62b0d6fc100992@linux.vnet.ibm.com>
Reviewed-by: Vasant Hegde <a6d753843573f7bfed5075c1e875b0be8007a0ab@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

shenki/skiboot,stewart-ibm/skiboot,open-power/skiboot,legoater/skiboot,open-power/skiboot,qemu/skiboot,mikey/skiboot,shenki/skiboot,qemu/skiboot,legoater/skiboot,open-power/skiboot,shenki/skiboot,qemu/skiboot,open-power/skiboot,mikey/skiboot,shenki/skiboot,open-power/skiboot,shenki/skiboot,stewart-ibm/skiboot,stewart-ibm/skiboot,legoater/skiboot,legoater/skiboot,legoater/skiboot,qemu/skiboot,mikey/skiboot,qemu/skiboot