dev_freq: devfreq_spdm: add null terminator to prevent OOB access

Add null terminator to end of buffered copied from user to prevent
over reading.

Change-Id: I80cfcb087ea2c335fd65d8fcdaf372c7d34a533d
Signed-off-by: David Dai <daidavid1@codeaurora.org>
Signed-off-by: phanendra yarra <pyarra@codeaurora.org>