staging: skein: Renames skein to skein_base

Renames skein.c to skein_base.c and skein.h to skein_base.h in
preparation for naming loadable module skein.ko

Signed-off-by: Eric Rost <eric.rost@mybabylon.net>
Reviewed-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>