staging: slicoss: free IO remapping on failure

Make sure iounmap is always called after ioremap when module loading
fails. Also remove a call to release_mem_region because that region is
never reserved in the first place.

Fixes the following issue reported by Coccinelle:
drivers/staging/slicoss/slicoss.c:3727:1-7: ERROR: missing iounmap; ioremap on line 3661 and execution via conditional on line 3677

Signed-off-by: Kristina Martšenko <kristina.martsenko@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>