iwlwifi: remove duplicate initialization

rate_mask is initialized again later so this can be removed.  Btw, if
rate_control_send_low(sta, priv_sta, txrc) returns false, that means
that "sta" is non-NULL.  That's why the second initialization of
rate_mask is a little simpler than the first.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>