wl18xx: align event mailbox with current fw

Some fields are missing from the event mailbox
struct definitions, which cause issues when
trying to handle some events.

Add the missing fields in order to align the
struct size (without adding actual support
for the new fields).

Reported-and-tested-by: Imre Kaloz <kaloz@openwrt.org>
Cc: stable@vger.kernel.org # 3.14+
Fixes: 028e7243 ("wl18xx: move to new firmware (wl18xx-fw-3.bin)")
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>