block: xen-blkfront: Demote kernel-doc abuses

Fixes the following W=1 kernel build warning(s):

 drivers/block/xen-blkfront.c:1960: warning: Function parameter or member 'dev' not described in 'blkfront_probe'
 drivers/block/xen-blkfront.c:1960: warning: Function parameter or member 'id' not described in 'blkfront_probe'
 drivers/block/xen-blkfront.c:1960: warning: expecting prototype for Allocate the basic(). Prototype was for blkfront_probe() instead
 drivers/block/xen-blkfront.c:2085: warning: Function parameter or member 'dev' not described in 'blkfront_resume'
 drivers/block/xen-blkfront.c:2085: warning: expecting prototype for or a backend(). Prototype was for blkfront_resume() instead
 drivers/block/xen-blkfront.c:2444: warning: wrong kernel-doc identifier on line:

Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: xen-devel@lists.xenproject.org
Cc: linux-block@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Link: https://lore.kernel.org/r/20210312105530.2219008-11-lee.jones@linaro.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>