floppy: check_events callback should not return a negative number

floppy_check_events() is supposed to return bit flags to say which
events occured. We should return zero to say that no event flags are
set.  Only BIT(0) and BIT(1) are used in the caller. And .check_events
interface also expect to return an unsigned int value.

However, after commit a0c80efe, it may return -EINTR (-4u).
Here, both BIT(0) and BIT(1) are cleared. So this patch shouldn't
affect runtime, but it obviously is still worth fixing.

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: a0c80efe ("floppy: fix lock_fdc() signal handling")
Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>