diag: Remove world read/write permissions from /dev/diag

/dev/diag has world read/write permissions which is considered
a security risk.  Remove these permissions.  Add new qcom_diag
group for access to /dev/diag

CRs-Fixed: 356415
(cherry picked from commit 14c85ec56fd47c5eb143617a3bd5a6e7ee39cb6e)

Change-Id: I536323a97e19a07bbd8eadffa0265d1955590285