greybus: manifest: really minor cleanups

This patch incorporates some very small cleanups to "manifest.c":
    - Rearrange code a bit in gb_manifest_parse() that ensures a
      manifest is big enough to hold a header.  If the manifest is
      exactly the size of a header, the error reported will now be
      "...must have 1 interface..." rather than "short manifest".
    - Fix the function comment for gb_manifest_parse_cports().
    - Use "an interface," not "a interface," and don't capitalize.
    - Delete some braces when getting interface product string.
    - A few other minor changes to comments and white space.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>