irqdomain: Drop pointless NULL check in virq_debug_show_one

data has been already derefenced unconditionally, so it's pointless to do a
NULL pointer check on it afterwards. Drop it.

[ tglx: Depersonify changelog. ]

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/20171112212904.28574-1-linux@rasmusvillemoes.dk