mtd: rawnand: do not execute nand_scan_ident() if maxchips is zero

Some driver (eg. docg4) will need to handle themselves the
identification phase. As part of the migration to use nand_scan()
everywhere (which will unconditionnaly call nand_scan_ident()), we add
a condition at the start of nand_scan_with_ids() to jump over
nand_scan_ident() if the maxchips parameters is zero, meaning that the
driver does not want the core to handle this phase.

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