[MTD] maps ixp2000: fix compile warnings in ixp2000 map driver

Fix two compile warnings that occur because of treating two
'unsigned long's as 'void *'s.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>