[MTD] maps/ixp4xx: kill some warnings

- Use map.virt instead of map.map_priv_1 since it has the correct type.
- Use readw/writew instead of dereferencing an ioremap'd cookie.
- Remove an unused variable.

Signed-off-by: David Vrabel <dvrabel@arcom.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>