changed pows to powfs

kernel.cu(152) (col. 8): error: calling a __host__
function("std::pow<int, double> ") from a __global__ function("short")
is not allowed

kernel.cu(152) (col. 8): error: calling a __host__
function("std::pow<int, double> ") from a __global__ function("short")
is not allowed

kernel.cu(161) (col. 8): error: calling a __host__
function("std::pow<int, double> ") from a __global__ function("short")
is not allowed

kernel.cu(163) (col. 8): error: calling a __host__
function("std::pow<int, double> ") from a __global__ function("short")
is not allowed

4 errors detected in the compilation of
"/tmp/tmpxft_0000489a_00000000-6_kernel.cpp1.ii".