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: I1788d655f048124c6b7cccee4345d8239743ada8
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>