virt: sev-guest: Pass the appropriate argument type to iounmap()

Fix a sparse warning in sev_guest_probe() where the wrong argument type is
provided to iounmap().

Fixes: fce96cf0

 ("virt: Add SEV-SNP guest driver")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/202207150617.jqwQ0Rpz-lkp@intel.com