mvsas: always iounmap resources

In case pci_resource_start() or pci_resource_len() reutrn 0, mvsas_ioremap
returns without doing an iounmap() of mvi->regs_ex.

Found by the cocinelle tool.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>