wlcore: split 18xx and 12xx scan mechanism

The scan APIs of 12xx and 18xx are totally different.
Use some common functions as much as possible (e.g.
for setting scan channels), but split scan.c into
chip-specific scan.c files, each implementing its
own scan mechanism.

(in other words - move most of the current wlcore's
scan.c into wl12xx, and implement a similar mechanism
in 18xx, according to the new api)

New wlcore ops are introduced in order to call the
chip-specific scan functions.

The template indices used for each scan (regular/scheduled)
are also different between the chips, so set the correct
indices used for each scan type after identifying the chip.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>