scsi: myrs: only build on little-endian platforms

Reading throught the new driver, I noticed that this cannot work on
big-endian CPUs, and the old DAC960 had exactly the same behavior.

To document this for the future, add a Kconfig dependency that prevents it
from being included in big-endian kernels.  Since the hardware is really
old and we never had a working driver on it for big-endian platforms,
it's unlikely to make a difference to users.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>