net: ieee802154: Add grounds for passive scan

Currently, as only CC2520 device is supported: it will loop over the 16
channels present in 2.4Ghz spectrum. However, this will need refinement
as soon as other spectrums will be in use: L2 layer could request
capabilites from the device in order to know which range to use.

Scan result is done one at a time for now. It's unlikely going to fit
all production use case, so it's going to change in the future.

Change-Id: I2fafec49eb5cbfca1a4f04107ba3b6511d7d956d
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>