ARCv2: cache: fix slc_entire_op: flush only instead of flush-n-inv

slc_entire_op with OP_FLUSH command also invalidates it.

This is a preventive fix as the current use of slc_entire_op is only
with OP_FLUSH_N_INV where the invalidate is required.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
[vgupta: fixed changelog]
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>