infiniband: ehca: Fix compiler warnings

Fix comp_task() to return void to match smp_hotplug_thread's thread_fn
member, and adjust indirection on the ->cpu_comp_threads per-CPU
member of the ehca_comp_pool structure.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>