brcmfmac: move firmware event related defines to fweh.h

The dhd.h file contained a number of definitions that are
related to events received from the firmware. Those are
processed and dispatched in the driver by fweh. Hence the
definitions are moved to its include file.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>