mmc: fsl_esdhc_spl: pre-PBL: fix determining U-Boot size

mmc: fsl_esdhc_spl: pre-PBL: fix determining U-Boot size

In pre-PBL header is stored size of code which BootROM copies from SD card
to L2/SRAM. This size has upper limit of L2 cache size. In most cases this
is size of U-Boot SPL or size of L2 cache.

Therefore this size in pre-PBL header cannot be used for determining size
of proper U-Boot.

So always use CONFIG_SYS_MMC_U_BOOT_SIZE for determining size of proper
U-Boot which stored on SD card.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>