irqchip/qcom: Fix u32 comparison with value less than zero

The comparison of u32 nregs being less than zero is never true since
nregs is unsigned. Fix this by making nregs a signed integer.

Fixes: f20cc9b0 ("irqchip/qcom: Add IRQ combiner driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: kernel-janitors@vger.kernel.org
Cc: Jason Cooper <jason@lakedaemon.net>
Link: https://lkml.kernel.org/r/20171117183553.2739-1-colin.king@canonical.com