wil6210: support VR mode

Allow wil6210 to configure the system in Virtual Reality (VR) mode.
When system in VR mode it should have a low latency, and a high
responsive to events in the system. Wil6210 will set the rx interrupt
moderation to lower threshold and it will drop packets when the
transmit queue is full.

New sysfs "vr_enable" can be used by user space to enable VR mode.
WMI_VR_CFG_CMDID is used to configure the FW into VR mode.

Change-Id: If9e0a5eb3323f1479d76b259c848310508e7009c
Signed-off-by: Ahmad Masri <amasri@codeaurora.org>