[MTD] NAND: Check command timeout

Check timeout while we wait for the command to finish. No worry about a
false result. This prevents deadlocking when detecting an unknown number
of chips and is useful for removable media too.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>