x86: Remove the useless branch in c_start()

Since 'cpu == -1' in cpumask_next() is legal, no need to handle
'*pos == 0' specially.

About the comments:

	/* just in case, cpu 0 is not the first */

A test with a cpumask in which cpu 0 is not the first has been
done, and it works well.

This patch will remove that useless branch to clean the code.

Signed-off-by: Michael Wang <wangyun@linux.vnet.ibm.com>
Cc: kjwinchester@gmail.com
Cc: borislav.petkov@amd.com
Cc: ak@linux.intel.com
Link: http://lkml.kernel.org/r/1348033343-23658-1-git-send-email-wangyun@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>