tipc: Eliminate duplication of media structures

Changes TIPC's list of registered media types from an array of media
structures to an array of pointers to media structures. This eliminates
the need to copy of the contents of the structure passed in during media
registration.

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