iwlagn: check change before commit RXON cmd

When setting rxon chain and filter, no need to commit RXON when the
chain flag is not changed. This reduces the number of RXON commands
we send down to uCode.

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