sf: move useful messages from debug to printf

sf: move useful messages from debug to printf

At the moment, the default SPI flash subsystem is quite terse.  Errors and
successes both result in a generic message.  So move the useful errors and
useful successes to printf output by default.

While we're here, also convert the messages to use print_size().

Signed-off-by: Mike Frysinger <vapier@gentoo.org>