wlcore: update events enum/struct to new fw api

The event mailbox in wl18xx has a different
(non-compatible) structure.

Create common functions in wlcore to handle the
events, and call them from the chip-specific
event mailbox parsers.

This way, each driver (wl12xx/wl18xx) extracts
the event mailbox by itself according to its
own structure, and then calls the common
wlcore functions to handle it.

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