[MTD] Cleanup of 'ioremap balanced with iounmap for drivers/mtd subsystem'

Updated version of patch, in response to comments from Francois Romieu
<romieu@fr.zoreil.com>

Remove gratuitous casts from iounmap and initialisation of variables.

Signed-off-by: Amol Lad <amol@verismonetworks.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>