drivers/xen: use helper macro __ATTR_RW

Use helper macro __ATTR_RW to define HYPERVISOR_ATTR_RW to make code more clear.
Minor readability improvement.

Remove extra whitespace [boris: added this comment]

Signed-off-by: zhanglianjie <zhanglianjie@uniontech.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20220305133823.158961-1-zhanglianjie@uniontech.com


Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>