ath9k_hw: Various trivial fixes for PAPRD

* Remove unneeded memset.

  All the values in the PAPRD gain table are filled, so there
  is no need to zero out the arrays.

* Use GFP_KERNEL in ar9003_paprd_create_curve

  This is called from the PAPRD work, so the atomic variant
  is not needed.

* Change return type of ar9003_paprd_setup_gain_table

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>