tipc: Avoid pointless masking of fragmented message identifier

Eliminates code that restricts a link's counter of its fragmented
messages to a 16-bit value, since the counter value is automatically
restricted to this range when it is written into the message header.

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