bdev: Fix DRBD error reporting for syncer rate

The function is supposed to return a list, not a string. Without this
patch the error message ends up as “C, a, n, ', t,  , c, h, a, n, g, e,
…”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>