mmc: Use EXT_CSD_EXTRACT_BOOT_PART() macro for extracting boot part

Mask macro PART_ACCESS_MASK filter out access bits of emmc register and
macro EXT_CSD_EXTRACT_BOOT_PART() extracts boot part bits of emmc register.
So use EXT_CSD_EXTRACT_BOOT_PART() when extracting boot partition.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>