mtd: spinand: Propagate the bus interface across core helpers

mtd: spinand: Propagate the bus interface across core helpers

For now all drivers provide SSDR variants only. When we add support for
ODTR modes, there will be a need to differentiate the type of variant we
target as well as the need to check if we support one or the other type
of operations.

Pass this parameter to lower level helpers, which for now is unused, in
order to simplify the patch introducing ODTR support.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>