Small cleanups.

Small cleanups.

Add some parentheses to make an expression clearer to people who haven't
memorized the table of C operator precedences.

Don't fiddle the nss variable in place; explicitly combine it with the
IS_TX value when we put it in the header, to make it clearer what's in
that header byte.

Change-Id: I870b892fb9dab2bc210956f923e0183f4e147989
Reviewed-on: https://code.wireshark.org/review/21530
Reviewed-by: Guy Harris <guy@alum.mit.edu>