wl18xx: fallback to default conf in case of invalid conf file

If the wl18xx-conf.bin file is missing or invalid (e.g. due
to recent driver change), fallback to default configuration
instead of failing driver load.

Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>