arm: mvebu: spl: Fix parsing SATA kwbimage

Despite the official specification, Marvell BootROM does not interpret
srcaddr from SATA image as number of sectors the beginning of the hard
drive, but as number of sectors relative to the main header.

To process SATA kwbimage and load U-Boot proper from it in the same way as
Marvell BootROM, it is needed to interpret srcaddr as relative offset to
the main header. This change fixes booting of U-Boot proper from SPL code
in SATA image.

Fixes: 2226ca17 ("arm: mvebu: Load U-Boot proper binary in SPL code based on kwbimage header")
Signed-off-by: Pali Rohár <pali@kernel.org>