staging: gasket: sysfs: remove unnecessary NULL check on device ptr

The device pointer passed into get_mapping() will never be NULL; the
check is unnecessary.

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