The MCE driver's helper functions were using postive values as error
codes.

This patch updates the functions to return negative values as error
codes instead. Some functions are updated to use the right error code.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Change-Id: I3e2ecc30a6272a357e1a22ec850543fde2a694f6