nvme: don't enable AEN if not supported

Avoid excuting set_feature command if there is no supported bit in
Optional Asynchronous Events Supported (OAES).

Fixes: c0561f82

 ("nvme: submit AEN event configuration on startup")
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Weiping Zhang <zhangweiping@didichuxing.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>