power: Fix coverity error

Properly null-terminate userspace input string. Otherwise, the subsequent
strsep() could continue off into arbitrary chunks of kmalloc() space
that aren't part of the original string buffer.

Change-Id: I3868dbcdd9df7e7172c001eb6bc41c605d48604b
Signed-off-by: Sai Gurrappadi <sgurrappadi@nvidia.com>
Reviewed-on: http://git-master/r/355578
Reviewed-by: Paul Walmsley <pwalmsley@nvidia.com>
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Diwakar Tundlam <dtundlam@nvidia.com>