platform/x86: intel_pmc_core: Avoid a u32 overflow

The register (SLP_S0_RES) at offset slp_s0_offset is a 32 bit register.
The pmc_core_adjust_slp_s0_step() could overflow the u32 value while
returning it after adjusting the step. Thus change to u64, this is
already accounted for in debugfs attribute (that wants to output a
64 bit value).

Signed-off-by: Rajat Jain <rajatja@google.com>
Acked-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>