msm_rmnet: Fix potential memory corruption

Usage of legacy string manipulation function sprintf could lead to
memory corruption as the number of bytes to be written to the
destination is not bounded. Replace sprintf with snprintf where the
number of bytes to be written is explicitly specified.

CRs-fixed: 672202
Change-Id: I77f8d1abe2067bee1c68d7a60a35866fced54b8c
Signed-off-by: Michael Vines <mvines@codeaurora.org>