ath9k_hw: fix an off-by-one error in the PDADC boundaries calculation

PDADC values were only generated for values surrounding the target
index, however not for the target index itself, leading to a minor
error in the generated curve.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>