wigig_sensing: add GET_NUM_AVAIL_BURSTS ioctl

WIGIG_SENSING_IOCTL_GET_NUM_AVAIL_BURSTS returns the number of available
bursts in the drivers buffer.

Change-Id: I474770a817393e1b64831886803d65a3f76d92e0
Signed-off-by: Gidon Studinski <gidons@codeaurora.org>
Signed-off-by: Alexei Avshalom Lazar <ailizaro@codeaurora.org>