[IA64] Check return from device_register() in cx_device_register()

The device_register() is declared with must_check, causing this:

arch/ia64/sn/kernel/tiocx.c: In function 'cx_device_register':
arch/ia64/sn/kernel/tiocx.c:210:17: warning: ignoring return value of 'device_register', declared with attribute warn_unused_result

Check the return value, and free resources if it fails.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>