- Fix bug on crashing when module unloaded but no chip present

- Add access to registers via sysfs (needed for accessing calibration
  register which is needed for a good stability of the RTC)

Signed-off: Christian Pellegrin <chripell@fsfe.org>
Tested-by: Christian Pellegrin <chripell@fsfe.org>
Cc: Chris Verges <chrisv@cyberswitching.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>