hwmon: (ibmpex) Release IPMI user if hwmon registration fails

Roel Kluin <12o3l@tiscali.nl> found a minor defect in the init code if
hwmon device registration fails.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>