gpio: Fix error checking in the function device_pca957x_init

This fixes error checking in the function device_pca957x_init
to properly check and return error code values from the calls
to the function pca953x_write_regs if they fail as to properly
signal callers when a error occurs due a failure when writing
registers for this gpio based device.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>