discover/platform-powerpc: Return bootdev error correctly

discover/platform-powerpc: Return bootdev error correctly

In one case get_ipmi_bootdev_ipmi() can return 0 on an error, which
leads to bootdev being treated as a valid bootdev despite being
uninitialised. If the planets line up correctly and bootdev is less than
or equal to IPMI_BOOTDEV_SETUP, Petitboot will incorrectly apply an IPMI
override.

Update the error return value in get_ipmi_bootdev_ipmi(), and properly
initialise bootdev.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>