staging: gasket: Remove stale pointers on error allocating attr array

If gasket_sysfs_create_mapping() hits errors allocating the attribute
array, remove stale pointers to device info from the mapping object.

Signed-off-by: Zhongze Hu <frankhu@chromium.org>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>