tiocx: check retval from bus_register()

Properly check return value from bus_register() and propagate it out of
tiocx_init() in case of failure.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Tony Luck <tony.luck@intel.com>