mtd: pxa3xx_nand: Simplify pxa3xx_nand_scan

This commit simplifies the initial configuration performed
by pxa3xx_nand_scan. No functionality change is intended.

This commit is taken from Linux:
'commit 154f50fbde53'
("mtd: pxa3xx_nand: Simplify pxa3xx_nand_scan")

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Ofer Heifetz <oferh@marvell.com>
Reviewed-by: Igal Liberman <igall@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Stefan Roese <sr@denx.de>