gpio: pca953x: propagate the errno from the chip_init functions

Initializing the chips may return with an error, but this error gets
dropped in probe(). Propagate this further to the driver core. Also,
simplify returning the error in one of the init functions.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>