ath9k_hw: get QCA953x WMAC revision via platform_data

The SREV register in the WMAC register space does not contain the chip
revision, so it needs to be passed in from the kernel.

With an updated kernel, this fixes tx gain table selection.

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