mmc: core: Annotate cmd_hdr as __le32

Commit f68381a7 (mmc: block: fix packed command header endianness)
correctly fixed endianness handling of packed_cmd_hdr in
mmc_blk_packed_hdr_wrq_prep.

But now, sparse complains about incorrect types:
drivers/mmc/card/block.c:1613:27: sparse: incorrect type in assignment (different base types)
drivers/mmc/card/block.c:1613:27:    expected unsigned int [unsigned] [usertype] <noident>
drivers/mmc/card/block.c:1613:27:    got restricted __le32 [usertype] <noident>
...

So annotate cmd_hdr properly using __le32 to make everyone happy.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Fixes: f68381a7 (mmc: block: fix packed command header endianness)
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>