iwlwifi: verify the antenna selection when receive fixed rate debugfs

When iwlwifi driver receive fixed rate debugfs command, validate the
antenna selection, if the selection is invalid, report the valid antenna
choice and do not set the rate scale table to fixed rate. Otherwise, set
the entire rate scale table to the fixed rate request by the user. this
validation can prevent sysassert happen in uCode

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>