MIPS: lantiq: Change variables to the same type as the source

A structure irq_data, irq_desc_get_irq() and irq_linear_revmap() use
a different type than defined in the lantiq ICU driver, which is using
signed integers. The substracted result should never be negative nor is
tested for that situation. Change it to unsigned.

Signed-off-by: Petr Cvek <petrcvekcz@gmail.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: hauke@hauke-m.de
Cc: john@phrozen.org
Cc: linux-mips@vger.kernel.org
Cc: openwrt-devel@lists.openwrt.org
Cc: pakahmar@hotmail.com