iio: adc: gyroadc: fix uninitialized return code

gcc-9 complains about a blatant uninitialized variable use that
all earlier compiler versions missed:

drivers/iio/adc/rcar-gyroadc.c:510:5: warning: 'ret' may be used uninitialized in this function [-Wmaybe-uninitialized]

Return -EINVAL instead here and a few lines above it where
we accidentally return 0 on failure.

Cc: stable@vger.kernel.org
Fixes: 059c53b3

 ("iio: adc: Add Renesas GyroADC driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>