cmd: mvebu/bubt: Fix parsing SDIO kwbimage

Despite the official specification, Marvell BootROM does not interpret
srcaddr from SDIO image as offset in number of sectors (like for SATA
image), but as offset in bytes (like for all other images except SATA).

To ensure that we do not store invalid SDIO image to the boot location
(read by the Marvell BootROM), we need to check that image is valid
and srcaddr is intepreted in bytes, in the same way as it is done by
Marvell BootROM.

This fixes rejecting valid and accepting invalid SDIO images by bubt command.

Fixes: 5a065349 ("cmd: mvebu/bubt: Check for A38x image data checksum")
Signed-off-by: Pali Rohár <pali@kernel.org>