mmc: Fix error in RPMB code

Since we do not build any board with CONFIG_SUPPORT_EMMC_RPMB , this
piece of code evaded conversion. Fix the following compiler error:

cmd/mmc.c: In function 'do_mmcrpmb':
cmd/mmc.c:316:32: error: 'struct blk_desc' has no member named 'part_num'
  original_part = mmc->block_dev.part_num;
                                ^

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Tom Rini <trini@konsulko.com>