OCL: setIdentity fix

Kernel with `kercn = 4 && cn = 1` works correct only with `rowsPerWI = 4`.
I believe that in case `kercn = 4, rowsPerWI = 1` kernel code will be complicated and maybe works slower.
test_modules=core
build_examples=OFF
check_regression=OCL*SetIdentity*
test_filter=OCL*SetIdentity*