btrfs: Make btrfs_find_device_by_devspec return btrfs_device directly

Instead of returning an error value and using one of the parameters for
returning the actual object we are interested in just refactor the
function to directly return btrfs_device *. Also bubble up the error
handling for the special BTRFS_ERROR_DEV_MISSING_NOT_FOUND value into
btrfs_rm_device. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>