mtd: pxa3xx_nand: fix nand detection issue

When keep_config is set, the detection would goes different routine.
That the driver would read out the setting which is set previously
by bootloader. While most bootloader keep the irq mask as off, and
current driver need all irq default open, keep_config behavior would
lead to no irq at all.

Signed-off-by: Lei Wen <leiwen@marvell.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Cc: stable@kernel.org [2.6.38+]
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>