disk: don't try search for partition type if already set

Block devices can already set partition type at initialization
stage, so, in this case is no point in searching for partition type.

Signed-off-by: Alexey Romanov <avromanov@salutedevices.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>