hw/sd/sdcard: Fix calculation of size when using eMMC boot partitions

hw/sd/sdcard: Fix calculation of size when using eMMC boot partitions

The sd_bootpart_offset() function calculates the *runtime* offset which
changes as the guest switches between accessing the main user data area
and the boot partitions by writing to the EXT_CSD_PART_CONFIG_ACC_MASK
bits, so it shouldn't be used to calculate the main user data area size.

Instead, subtract the boot_part_size directly (twice, as there are two
identical boot partitions defined by the eMMC spec).

Suggested-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Fixes: c8cb19876d3e ("hw/sd/sdcard: Support boot area in emmc image")
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Cédric Le Goater <clg@redhat.com>