bluez5: Point hciattach bcm43xx firmware search path to /lib/firmware

Currently the hciattach bcm43xx firmware loader looks up the firmware
blob in /etc/firmware . Change this to /lib/firmware instead, so that
the path is consistent with Linux kernel which also looks up firmware
for the WiFi part in /lib/firmware .

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>