tipc: Eliminate redundant masking in message header routines

Gets rid of unnecessary masking in two routines that set TIPC message
header fields. (The msg_set_bits() routine already takes care of
masking the new value to the correct size.)

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