Guessing the size of an attached harddisk (access via x86 BIOS) was needed
due to the fact, barebox can't query this information from the BIOS easily.

But with the SD/MMC cards, there will be a second user of the generic disk
handling routines. And with this media it is very easy to know its size.

This patch provides a workaround to keep the guessing feature if the size of
the registered disk is 0. If it is not 0, the given value will be used instead.

Note: This is in preparation to add MCI card support, which can be handled
like a disk drive.

Signed-off-by: Juergen Beisert <jbe@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>