iwlagn: iwl-agn-rs: remove unnecessary null check for sta and lq_sta

both sta and lq_sta are guaranteed to be not null in the
calling function so we don't need to check them here.

Signed-off-by: Greg Dietsche <Gregory.Dietsche@cuw.edu>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>