MLBEDSW-7527: Mean operator output diff

Mean operators with height larger than 64 are reshaped but the IFM shape
was then reset to the original value, causing an output diff.

Signed-off-by: Rickard Bolin <rickard.bolin@arm.com>
Change-Id: I3a89d4efac53173cbd6fe0a5c0542e028bed42ad