tipc: remove abstraction for link_max_pkt

This is just a straight return of a field; there is no
value in the abstraction of hiding it behind a function.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>