bridge: Try to make the GRAB_OUR_PACKETS macro a bit more readable

• Let the compiler use constant folding to eliminate conditionals.
• Fix some inconsistent whitespace.

No functional change intended.

Reviewed by: zlei
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D38410

(cherry picked from commit 82bbdde4ebc9deb30d3a05ddec4d76f0a88df881)