PCI/ASPM: Declare threshold_ns as u32, not u64

aspm_calc_l1ss_info() computes l1_2_threshold in microseconds as:

  l1_2_threshold = 2 + 4 + t_common_mode + t_power_on;

where t_common_mode is at most 255us:

  PCI_L1SS_CAP_CM_RESTORE_TIME  0x0000ff00   <-- 8 bits; <256us

and t_power_on is at most 31 * 100us = 3100us:

  PCI_L1SS_CAP_P_PWR_ON_VALUE   0x00f80000   <-- 5 bits; <32
  PCI_L1SS_CAP_P_PWR_ON_SCALE   0x00030000   <-- *2us, *10us, or *100us

So l1_2_threshold is at most 2 + 4 + 255 + 3100 = 3361, which means
threshold_ns is at most 3361 * 1000 = 3361000, which easily fits in a
u32.

Declare threshold_ns as u32, not u64.  This fixes a Coverity warning.

Addresses-Coverity-ID: 1462501
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
[bhelgaas: changelog]
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>