ARM: dts: msm: remove incorrect volume down setting

Remove incorrect setting of volume down key which causes msm8909
QRD SKUQ board enter into safe mode every time.

CRs-Fixed: 1058752
Change-Id: I1791b5c4371ef0f06665300502619010d67f0be2
Signed-off-by: Xiaogang Cui <xiaogang@codeaurora.org>
Signed-off-by: c_yinbin <yinbinzhang@codeaurora.org>