i2c: pca-platform: use device managed allocations

Switch to using the devm_ APIs and remove the now unnecessary error
handling and most of the device removal code.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
[wsa: adapted error handling I added in previous patch]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>