rtc: rtc-davinci: return correct error code if rtc_device_register() fails

rtc_device_register() returns a pointer containing error code in case
of error.  Use that in the error return.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Miguel Aguilar <miguel.aguilar@ridgerun.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>