x86/pmc_atom: Silence shift wrapping warnings in pmc_sleep_tmr_show()

I don't know if we really need 64 bits here but these variables are
declared as u64 and it can't hurt to cast this so we prevent any shift
wrapping.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Aubrey Li <aubrey.li@linux.intel.com>
Link: http://lkml.kernel.org/r/20140801082715.GE28869@mwanda
Signed-off-by: H. Peter Anvin <hpa@zytor.com>