brcm80211: Allow trace support to be enabled separately from debug

Since the runtime overhead of trace support is small when tracing is
disabled, users may be interested in turning on trace support while
leaving other debug features off. Add a new config option named
CONFIG_BRCM_TRACING for this purpose.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Tested-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>