wlcore: Fix config firmware loading issues

Booting multiple wl12xx and wl18xx devices using the same rootfs is
a pain. You currently have to symlink the right nvs file depending
on the wl12xx type.

For example, with wl1271-nvs.bin being a symlink to wl127x-nvs.bin
by default and trying to bring up a wl128x based device:

wlcore: ERROR nvs size is not as expected: 1113 != 912
wlcore: ERROR NVS file is needed during boot
wlcore: ERROR NVS file is needed during boot
wlcore: ERROR firmware boot failed despite 3 retries

Note that wl18xx uses a separate config firmware wl18xx-conf.bin
that can be generated with tools using the following two git repos:

git.ti.com/wilink8-wlan/18xx-ti-utils
git.ti.com/wilink8-wlan/wl18xx_fw

So let's not configure the nvs file for wl18xx as it's not needed
AFAIK. If it turns out that we also need the nvs file for wl18xx,
we can just add it to the config firmware data for wl18xx.

Let's fix the issue by using the chip specific config firmware
data, and make sure we produce understandable warnings if something
is missing.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>