ath6kl: Lower SDIO pad drive strength for hw2.1.1 board

Without this change, SDIO shuts down due to CRC error during
data communication to the firmware in some of the platform.
for example, scan request issued to the firmware doesn't
return scan completed events and cause the socket interface
to always return -16 (device busy).

SDIO pad drive strength should be reduced for hw2.1.1 board
to avoid such errors.

Signed-off-by: Raja Mani <rmani@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>