mtd: rawnand: mxc: remove useless checks in GET/SET_FEATURES functions

All the calls to the chip's hooks ->get/set_features() go through
the core's wrappers nand_get/set_features() that already do the
necessary checks about feature support. Remove these
checks from the mxc's functions.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>