MLBEDSW-10247: Fix non-8-bit transpose implementation

While Ethos-U55 does not support 32-bit transposing, it appears
that a previous approach that treats everything as 8-bit quantities
works for the most common transpose cases (NHWC, NHCW).

 - Restore the treat-as-8-bit transpose path for Ethos-U55.
 - Fixes bad slice address offsets when source tensor is unsliced.

Signed-off-by: Philip Hall <philip.hall@arm.com>
Change-Id: I2e28db217c50ce37b34e5265402acef318fdb873