msm: ipa: prevent string buffer overflows

On rmnet_ipa_set_data_quota() API, add the
string terminator to prevent vulnerability
of string buffer overflows on debug prints.

Change-Id: Ie669f6606f76b9006bce4edd0c6d04aef9cfb600
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>