iio: adc: mcp3422: fix locking on error path

iio: adc: mcp3422: fix locking on error path

Reading from the chip should be unlocked on error path else the lock
could never being released.

Fixes: eb17237e395a ("iio: adc: Add driver for Microchip MCP3422/3/4 high resolution ADC")
Fixes: 19cf74fd8b26 ("iio: adc: mcp3422: fix locking scope")
Acked-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Link: https://lore.kernel.org/r/20200901093218.1500845-1-angelo.compagnucci@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>