ath9k: fix a crash in the PA predistortion apply function

When updating the PAPRD table in hardware, PAPRD itself needs to be
disabled first, otherwise the hardware can throw a data bus error,
which upsets at least some platforms.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>