iwlagn: check ready in iwlagn_bss_info_changed()

In function iwlagn_bss_info_changed(), we need to check if the driver
is ready before doing real work. Also, the previously put WARN() is
removed because the vif is not guaranteed to be valid. uCode restart
routine will clear the vif.

Signed-off-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>