UBI: use own macros for the layout volume

This is a minor nicification: UBI_LAYOUT_VOLUME_TYPE and
UBI_LAYOUT_VOLUME_ALIGN are currently defined but not used -
use them.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>