use sizeof to avoid double use of a length define

use sizeof to avoid double use of a length define

Only a cosmetic change: the *_LEN defines are already used
when defining the buffer. Using sizeof() ensures that the length
stays consistent, even if the definition is modified.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Reviewed-by: Anatoly Burakov <anatoly.burakov@intel.com>