ath10k: pci: use mutex for diagnostic window CE polling

ath10k: pci: use mutex for diagnostic window CE polling

The DIAG copy engine is only used via polling, but it holds a spinlock
with softirqs disabled. Each iteration of our read/write loops can
theoretically take 20ms (two 10ms timeout loops), and this loop can be
run an unbounded number of times while holding the spinlock -- dependent
on the request size given by the caller.

As of commit 9706ffdceacc ("ath10k: download firmware via diag Copy
Engine for QCA6174 and QCA9377."), we transfer large chunks of firmware
memory using this mechanism. With large enough firmware segments, this
becomes an exceedingly long period for disabling soft IRQs. For example,
with a 500KiB firmware segment, in testing QCA6174A, I see 200 loop
iterations of about 50-100us each, which can total about 10-20ms.

In reality, we don't really need to block softirqs for this duration.
The DIAG CE is only used in polling mode, and we only need to hold
ce_lock to make sure any CE bookkeeping is done without screwing up
another CE. Otherwise, we only need to ensure exclusion between
ath10k_pci_diag_{read,write}_mem() contexts.

This patch moves to use fine-grained locking for the shared ce_lock,
while adding a new mutex just to ensure mutual exclusion of diag
read/write operations.

Tested on QCA6174A, firmware version WLAN.RM.4.4.1-00132-QCARMSWPZ-1.

Fixes: 9706ffdceacc ("ath10k: download firmware via diag Copy Engine for QCA6174 and QCA9377.")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>