(ad7414) Return proper error code for ad7414_probe()

Return proper error if i2c_check_functionality reports
the adapter does not support the capability we need.

Also remove unneeded initialization for err variable.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Sean MacLennan <smaclennan@pikatech.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>