mtd: nand: socrates: use nand_scan() for nand_scan_ident/tail() combo

For this driver, there is nothing between nand_scan_ident() and
nand_scan_tail().  They can be merged into nand_scan().

Also, nand_scan() returns an appropriate error value when it fails.
Use it instead of the fixed error code -ENXIO.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>