command len is set too early in the code, since when building
AMSDU, the size changes. This causes the byte count table to
have the wrong size.

Fixes: a0ec0169 ("iwlwifi: support new tx api")
Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>