hwmon: (ds1621) Clean up register access

Fix a few oddities in how the ds1621 driver accesses the registers:
* We don't need a wrapper to access the configuration register.
* Check for error before calling swab16. Error checking isn't
  complete yet, but that's a start.
* Device-specific read functions should never be called during
  detection, as by definition we don't know what device we are talking
  to at that point.
* Likewise, don't assume that register reads succeed during detection.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Aurelien Jarno <aurelien@aurel32.net>