wil6210: support configurable board file via sysfs

Add a sysfs allowing to override the board file containing
the RF characteristics of the 11ad device.
The new board file will be used the next time the interface
is brought up.
This change is intended to support platforms with multiple
11ad devices connected. Each 11ad device can be connected
to a different antenna module and the HW cannot automatically
detect the type of antenna connected, so the board file must
be explicitly specified for each device.

Change-Id: I40746032d9314057c64575bca96f3445816e5457
Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Lior David <liord@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>