libflash/mbox-flash: only wait for MBOX_DEFAULT_POLL_MS if busy
libflash/mbox-flash: only wait for MBOX_DEFAULT_POLL_MS if busy

This makes the mbox unit test run 300x quicker and seems to
shave about 6 seconds from boot time on Witherspoon.

Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

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