mbox: Reduce default BMC timeouts
mbox: Reduce default BMC timeouts

Rebooting a BMC can take 70 seconds. Skiboot cannot possibly spin for
70 seconds waiting for a BMC to come back. This also makes the current
default of 30 seconds a bit pointless, is it far too short to be a
worse case wait time but too long to avoid hitting hardlockup detectors
and wrecking havoc inside host linux.

Just change it to three seconds so that host linux will survive and
that, reads and writes will fail but at least the host stays up.

Also refactored the waiting loop just a bit so that it's easier to read.

Reported-by: Pridhiviraj Paidipeddi <fd8ea14509c256cbd04eaa4ee745a213945e70a1@linux.vnet.ibm.com>
Tested-by: Pridhiviraj Paidipeddi <fd8ea14509c256cbd04eaa4ee745a213945e70a1@linux.vnet.ibm.com>
Signed-off-by: Cyril Bur <6ce38dade8c4b52fd55c193fae70fd1d1275c3c3@au1.ibm.com>
Reviewed-by: Nicholas Piggin <24829078df5003d9dd1dd871ca2aeb0b1cf95ef4@gmail.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

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