vdpa: Follow kdoc comment style

Follow comment style mentioned in the Writing kernel-doc document [1].

Following warnings are fixed.
$ scripts/kernel-doc -v -none include/linux/vdpa.h
include/linux/vdpa.h:11: warning: missing initial short description on line:
 * vDPA callback definition.
include/linux/vdpa.h:11: info: Scanning doc for vDPA
include/linux/vdpa.h:15: warning: cannot understand function prototype: 'struct vdpa_callback '
include/linux/vdpa.h:21: warning: missing initial short description on line:
 * vDPA notification area
include/linux/vdpa.h:21: info: Scanning doc for vDPA
include/linux/vdpa.h:25: warning: cannot understand function prototype: 'struct vdpa_notification_area '
include/linux/vdpa.h:31: warning: missing initial short description on line:
 * vDPA vq_state definition
include/linux/vdpa.h:31: info: Scanning doc for vDPA
include/linux/vdpa.h:34: warning: cannot understand function prototype: 'struct vdpa_vq_state '
include/linux/vdpa.h:41: info: Scanning doc for vDPA device
include/linux/vdpa.h:51: warning: cannot understand function prototype: 'struct vdpa_device '
include/linux/vdpa.h:62: info: Scanning doc for vDPA IOVA range
include/linux/vdpa.h:66: warning: cannot understand function prototype: 'struct vdpa_iova_range '
include/linux/vdpa.h:72: info: Scanning doc for vDPA_config_ops
include/linux/vdpa.h:203: warning: cannot understand function prototype: 'struct vdpa_config_ops '
include/linux/vdpa.h:270: info: Scanning doc for vdpa_driver
include/linux/vdpa.h:275: warning: cannot understand function prototype: 'struct vdpa_driver '
include/linux/vdpa.h:347: info: Scanning doc for vdpa_mgmtdev_ops
include/linux/vdpa.h:360: warning: cannot understand function prototype: 'struct vdpa_mgmtdev_ops '

After this fix:

scripts/kernel-doc -v -none include/linux/vdpa.h
include/linux/vdpa.h:11: info: Scanning doc for struct vdpa_calllback
include/linux/vdpa.h:21: info: Scanning doc for struct vdpa_notification_area
include/linux/vdpa.h:31: info: Scanning doc for struct vdpa_vq_state
include/linux/vdpa.h:41: info: Scanning doc for struct vdpa_device
include/linux/vdpa.h:62: info: Scanning doc for struct vdpa_iova_range
include/linux/vdpa.h:72: info: Scanning doc for struct vdpa_config_ops
include/linux/vdpa.h:270: info: Scanning doc for struct vdpa_driver
include/linux/vdpa.h:347: info: Scanning doc for struct vdpa_mgmtdev_ops

[1] https://www.kernel.org/doc/html/latest/doc-guide/kernel-doc.html

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20210406170457.98481-2-parav@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>