Fix warning: writing to APSR without specifying a bitmask is deprecated

Reference: 2.1.3. Core registers - Cortex-M3 Devices Generic User Guide

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0552a/CHDBIBGJ.html
kernel.c: replace interrupt number variable type from unsigned int to int

The later comparision compares interrupt number with a negtive number.
That means the variable should be a signed integer rather than unsigned
integer.