ath9k: Fix regression in PA calibration

The commit "ath9k: Fix bugs in programming registers during PA CAL"
removed a REG_READ of 0x7834. This resulted in incorrect
computation of the subsequent value to be written in RF2G6.

This patch fixes the regression by re-adding the REG_READ.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>