mtd: rawnand: Remove nand_do_read() prototype from rawnand.h

nand_do_read() is a static function implemented in nand_base.c. There's
no good reason to expose its prototype in rawnand.h.

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