mmc: block: fix packed command header endianness

The code that fills packed command header assumes that CPU runs in
little-endian mode. Hence the header is malformed in big-endian mode
and causes MMC data transfer errors:

[  563.200828] mmcblk0: error -110 transferring data, sector 2048, nr 8, cmd response 0x900, card status 0xc40
[  563.219647] mmcblk0: packed cmd failed, nr 2, sectors 16, failure index: -1

Convert header data to LE.

Signed-off-by: Taras Kondratiuk <takondra@cisco.com>
Fixes: ce39f9d1 ("mmc: support packed write command for eMMC4.5 devices")
Cc: <stable@vger.kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>