mtd: pxa3xx_nand: Simplify pxa3xx_nand_scan

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

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>