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>
diff --git a/board/purism/librem5/librem5.c b/board/purism/librem5/librem5.c
index caa0265..386ed1b 100644
--- a/board/purism/librem5/librem5.c
+++ b/board/purism/librem5/librem5.c
@@ -41,7 +41,7 @@
 #if IS_ENABLED(CONFIG_LOAD_ENV_FROM_MMC_BOOT_PARTITION)
 uint board_mmc_get_env_part(struct mmc *mmc)
 {
-	uint part = (mmc->part_config >> 3) & PART_ACCESS_MASK;
+	uint part = EXT_CSD_EXTRACT_BOOT_PART(mmc->part_config);
 
 	if (part == 7)
 		part = 0;