rtc: stmp3xxx: print message on error

stmp3xxx_wdt_register() can fail as platform_device_alloc() or
platform_device_add() can fail. But when it fails it failed silently.
Lets print out an error message on failure so that user will atlest
know that there was some error.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>