genirq/matrix: Make - vs ?: Precedence explicit

Noticed with a Clang build. This improves the readability of the ?:
expression, as it has lower precedence than the - expression. Show
explicitly that - is evaluated first.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20171122205645.GA27125@beast