tipc: Streamline media registration error checking

Simplifies error handling performed during media registration, since
TIPC no longer supports the dynamic addition of new media types that
are potentially error-prone. These simplifications include the following:

1) No longer check for premature registration of a new media type.
2) No longer check for negative link priority values (which was pointless
   since such values are unsigned, and could cause a compiler warning).
3) No longer generate a warning describing the exact cause of any
   registration failure (just warns that overall registration failed).

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>