IB/ipath: Correct ipath_verbs_register_sysfs() error handling

ipath_verbs_register_sysfs() never returned the correct error
code from device_create_file and never cleaned up from a failure.
Additionally, the caller of ipath_verbs_register_sysfs() doesn't
return the correct "ret" value.

This patch resolves all of these issues.

Reported-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>