xen-blkfront: Fix handling of non-supported operations

This patch fixes the following sparse warnings:

drivers/block/xen-blkfront.c:916:45: warning: incorrect type in argument 2 (different base types)
drivers/block/xen-blkfront.c:916:45:    expected restricted blk_status_t [usertype] error
drivers/block/xen-blkfront.c:916:45:    got int [signed] error
drivers/block/xen-blkfront.c:1599:47: warning: incorrect type in assignment (different base types)
drivers/block/xen-blkfront.c:1599:47:    expected int [signed] error
drivers/block/xen-blkfront.c:1599:47:    got restricted blk_status_t [usertype] <noident>
drivers/block/xen-blkfront.c:1607:55: warning: incorrect type in assignment (different base types)
drivers/block/xen-blkfront.c:1607:55:    expected int [signed] error
drivers/block/xen-blkfront.c:1607:55:    got restricted blk_status_t [usertype] <noident>
drivers/block/xen-blkfront.c:1625:55: warning: incorrect type in assignment (different base types)
drivers/block/xen-blkfront.c:1625:55:    expected int [signed] error
drivers/block/xen-blkfront.c:1625:55:    got restricted blk_status_t [usertype] <noident>
drivers/block/xen-blkfront.c:1628:62: warning: restricted blk_status_t degrades to integer

Compile-tested only.

Fixes: commit 2a842aca ("block: introduce new block status code type")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Roger Pau Monné <roger.pau@citrix.com>
Cc: <xen-devel@lists.xenproject.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>