USB: serial: fix compare_const_fl.cocci warnings

Move constants to the right of binary operators where it increases
readability.

Generated by: scripts/coccinelle/misc/compare_const_fl.cocci

Signed-off-by: Mathieu OTHACEHE <m.othacehe@gmail.com>
[johan: drop some chunks and fix others, amend commit message ]
Signed-off-by: Johan Hovold <johan@kernel.org>