dm: cbfs: Fix handling of invalid type

The comment for file_cbfs_type() says that it returns 0 for an invalid type.
The code appears to check for -1, except that it uses an unsigned variable
to store the type. This results in a warning on 64-bit machines.

Adjust it to make the meaning clearer. Continue to handle the -1 case since
it may be needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>