iwlwifi: take a valid antenna upon rate scale init

This patch selects a valid antennae upon rate scale init. This solves a
SYSASSERT complaining that the driver is setting a non valid antenna in
the LQ CMD.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>