ath10k: fix invalid STS_CAP_OFFSET_MASK

The 10.4 firmware defines this as a 3-bit field, as does the
mac80211 stack.  The 4th bit is defined as CONF_IMPLICIT_BF
at least in the firmware header I have seen.  This patch
fixes the ath10k wmi header to match the firmware.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>