MLBEDSW-10594: Fix OFM quantization for decomped DepthwiseConv2D

When DepthwiseConv2D has depth multiplier >1, it's decomposed into
one op per depthwise multiplier. Weights and biases are sliced as
expected, but the per-channel OFM quantization also must be sliced.

Signed-off-by: Johan Gunnarsson <johan.gunnarsson@arm.com>
Change-Id: Ie7134987de9bb77dad99eb2322a2c8615e3e5471