spl: Change printf to puts for "Unsupported boot-device"

Microblaze currently doesn't use printf in SPL. So this one line was the only
reference to it and resulted in the printf functionality to be pulled in.
Exceeding the 4k size limit. Lets change the printf back to puts so that
Microblaze is fixed again. The only drawback is that the detected boot-device
number will not be printed. But this message alone should be helpful enough
to get an idea where the boot process is broken.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>