ath9k_hw: clean up RF Bank6 handling on AR5416/AR91xx

There are two sets of initvals for this RF bank, one with TPC support and
one without.
The TPC one always gets used, so remove the other one to avoid confusion.

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