mtd: omap2: fix module loading

Unloading the omap2 nand driver missed to release the memory region which will
result in not being able to request it again if one want to load the driver
later on.

This patch fixes following error when loading omap2 module after unloading:
---8<---
~ $ rmmod omap2
~ $ modprobe omap2
[   37.420928] omap2-nand: probe of omap2-nand.0 failed with error -16
~ $
--->8---

This error was introduced in 67ce04bf which
was the first commit of this driver.

Signed-off-by: Andreas Bießmann <andreas@biessmann.de>
Cc: stable@vger.kernel.org
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>