ath9k_hw: fix PA predistortion HT40 mask

The commit 'ath9k_hw: Disable PAPRD for rates with low Tx power' changed
the code that sets the PAPRD rate masks to use only either the HT20 mask
or the HT40 mask. This is wrong, as the hardware can still use HT20 rates
even when configured for HT40, and the operating channel mode does not
affect PAPRD operation.
The register for the HT40 rate mask is applied as a mask on top of the
other registers to selectively disable PAPRD for specific rates on HT40
packets only.
This patch changes the code back to the old behavior which matches the
intended use of these registers. While with current cards this should not
make any practical difference (according to Atheros, the HT20 and HT40
mask should always be equal), it is more correct that way, and maybe
the HT40 mask will be used for some rare corner cases in the future.

Cc: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>