ath10k: add diag_read() to hif ops

diag_read() is used for reading from firmware memory via the diagnose window.
First user will be cal_data debugfs file.

To serialise diagnostic window access and make it safe to use while firmware is
running take ce_lock both in ath10k_pci_diag_write_mem() and
ath10k_pci_diag_read_mem(). Because of that all the CE calls had to be changed
to _nolock variants.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>