ath10k: decouple ath10k_pci_start_ce()

The function did a couple of things: it allocated
CE completions, registered CE callbacks and
enabled CE interrupts through HW registers.

This cannot be so. Split the function into one
that allocates CE completions and the other one
that starts off CE operation.

This is required for future legacy shared
interrupt handling.

This also fixes possible memory leak if post rx
failed.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>