ath10k: prevent using invalid ringbuffer indexes

If the device is removed and hotplug fails
ioread32() will return 0xFFFFFFFF. In that case
reading ringbuffer during device bringup led to
out-of-bounds addressing of a ringbuffer array
that in turn led to a paging failure.

This could be reproduced by the following:
 * boot without acpi/prevent hotplug from working
 * insert and manually detect (pci rescan) the device
 * remove the device physically
 * load ath10k driver
 * kernel crashed

Ringbuffer index reading is now protected by using
an appropriate mask to prevent addressing an
invalid array index.

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