ath10k: split ce initialization and allocation

Definitions by which copy engine structure are
allocated do not change so it doesn't make much
sense to re-create those structures each time
device is booted (e.g. due to firmware recovery).

This should decrease chance of memory allocation
failures.

While at it remove per_transfer_context pointer
indirection. The array has been trailing the copy
engine ringbuffer structure anyway. This also
saves pointer size worth of bytes for each copy
engine ringbuffer.

Reported-By: Avery Pennarun <apenwarr@gmail.com>
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>