bootdev: Fix the case where the driver ops field is null.

bootdev: Fix the case where the driver ops field is null.

In the case where a bootdev does not have a custom get_bootflow function
but instead relies on default_get_bootflow to provide one,
bootdev_get_bootflow was not handling the case where ops was simply not
set. Restructure the function to check for "ops && ops->get_bootflow"
and add appropriate log_debug calls for both cases.

Signed-off-by: Tom Rini <trini@konsulko.com>