wil6210: added sysfs file for FTM calibration

In fine timing measurements, the calculation is affected by
2 parts: timing of packets over the air, which is platform
independent, and platform-specific delays, which are dependent
on things like antenna cable length and type.
Add a sysfs file which allows to get/set these platform specific
delays, separated into the TX and RX components.
There are 2 key scenarios where the file can be used:
1. Calibration - start with some initial values (for example,
the default values at startup), make measurements at a known
distance, then iteratively change the values until the
measurement results match the known distance.
2. Adjust the delays when platform starts up, based on known
values.

Change-Id: If77667f2d80ddce3d5e5818e2447a2d05dd06034
Signed-off-by: Lior David <liord@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
[ailizaro@codeaurora.org: trivial conflict fixes]
Signed-off-by: Alexei Avshalom Lazar <ailizaro@codeaurora.org>
[liord@codeaurora.org: fix merge conflicts, SPDX license]
Signed-off-by: Lior David <liord@codeaurora.org>