wlcore: rename wl12xx.h to wlcore_i.h

The wl12xx.h file now contains mostly definitions that are internal to
wlcore.  Still, some things need to me moved to the public header, so
for now we keep including it in wlcore.h.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>