drivers: xen: gnttab: use correct struct for grant frames unmapping

Previously the driver's 'gnttab_unmap_refs()' signature used incorrect
struct - the same one that is used for mapping. Since 'host_addr'
membber, that is used to point to required frame is first in both
structures it somehow worked.

Fix mistake and use 'struct gnttab_unmap_grant_ref' for grant frames
unmapping hypercalls.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>