drm/amd/amdgpu: Fix warnings in amdgpu_irq.c

checkpatch reports:

WARNING: Prefer 'unsigned int' to bare use of 'unsigned'
WARNING: braces {} are not necessary for any arm of this statement
+               if (nvec <= 0) {
[...]
+               } else {
[...]
WARNING: Block comments use a trailing */ on a separate line

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>