flash/stmsmi: fix byte order for big-endian host

flash/stmsmi: fix byte order for big-endian host

The original code was written for and tested on little-endian
host only.

Rewrite it to be independent by host endianess.
Not tested on real HW; I don't own anymore a SPEAr device.

Change-Id: I2f427a804693f56cb9dea4936c525eb814c48c28
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reported-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4778
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>