parisc: Fix vmap memory leak in ioremap()/iounmap()

Sven noticed that calling ioremap() and iounmap() multiple times leads
to a vmap memory leak:
	vmap allocation for size 4198400 failed:
	use vmalloc=<size> to increase size

It seems we missed calling vunmap() in iounmap().

Signed-off-by: Helge Deller <deller@gmx.de>
Noticed-by: Sven Schnelle <svens@stackframe.org>
Cc: <stable@vger.kernel.org> # v3.16+