iwmc3200wifi: fix fragmentation threshold setting

We were sending the fragmentation threshold value to the wrong table,
causing an LMAC assert when setting it from wext.

Signed-off-by: Samuel Ortiz <samuel.ortiz@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>