ath10k: allow setting coverage class

Unfortunately ath10k does not generally allow modifying the coverage class
with the stock firmware and Qualcomm has so far refused to implement this
feature so that it can be properly supported in ath10k. If we however know
the registers that need to be modified for proper operation with a higher
coverage class, then we can do these modifications from the driver.

This is a hack and might cause subtle problems but as it's not enabled by
default (only when user space changes the coverage class explicitly) it should
not cause new problems for existing setups. But still this should be considered
as an experimental feature and used with caution.

This patch implements the support for first generation cards (QCA9880, QCA9887
and so on) which are based on a core that is similar to ath9k. The registers
are modified in place and need to be re-written every time the firmware sets
them. To achieve this the register status is verified after certain WMI events
from the firmware.

The coverage class may not be modified temporarily right after the card
re-initializes the registers. This is for example the case during scanning.

Thanks to Sebastian Gottschall <s.gottschall@dd-wrt.com> for initially
working on a userspace support for this. This patch wouldn't have been
possible without this documentation.

Signed-off-by: Benjamin Berg <benjamin@sipsolutions.net>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fit.fraunhofer.de>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>