hwmon: (ibmaem) Fix error paths

I am under the impression that error paths in functions
aem_init_aem1_inst() and aem_init_aem2_inst() are incorrect. In
several cases, the function returns 0 on error, which I suspect is
not intended. Fix this by properly tracking error codes.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Darrick J. Wong <djwong@us.ibm.com>
Acked-by: Guenter Roeck <guenter.roeck@ericsson.com>