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>