common: block: fix compiler error with CONFIG_FASTBOOT_FLASH_MMC_DEV

This fixes the following compiler error:

common/fb_mmc.c: In function ‘fb_mmc_erase’:
common/fb_mmc.c:209:17: error: ‘struct blk_desc’ has no member named
‘block_erase’

Signed-off-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>

common/fb_mmc.c

@@ -191,7 +191,7 @@ void fb_mmc_erase(const char *cmd)
 	printf("Erasing blocks " LBAFU " to " LBAFU " due to alignment\n",
 	       blks_start, blks_start + blks_size);
 
-	blks = dev_desc->block_erase(dev_desc, blks_start, blks_size);
+	blks = blk_derase(dev_desc, blks_start, blks_size);
 	if (blks != blks_size) {
 		error("failed erasing from device %d", dev_desc->devnum);
 		fastboot_fail("failed erasing from device");